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  • 251.
    Prokhorov, Dmitry
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Upper bounds on matter-antimatter admixture from gamma-ray observations of colliding clusters of galaxies with the Fermi Large Area Telescope2015Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, nr 8, artikel-id 083002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We examined whether significant constraints on matter-antimatter admixture can be derived from gamma-ray observations of colliding clusters of galaxies with Fermi-LAT. We selected ten known systems of colliding clusters of galaxies for the analysis and computed the upper bounds on matter-antimatter admixture in these systems, which range from 7×10−9 to 2×10−6. This allowed us to exclude a symmetric universe on scales of order ∼20  Mpc at the confidence level of 99.9%. Adopting the number of systems of colliding galaxy clusters from the Marenostrum Universe cosmological simulation, we checked if the Fermi-LAT second source catalog contains a sufficient number of gamma-ray sources to provide us with the required number of sources possibly associated with pp¯annihilation from cluster-anticluster collisions. We found that a matter-antimatter-symmetric universe is strongly ruled out on scales of order ∼20  Mpc if a matter-antimatter admixture in these bullet-like systems is of f≳10−5, and on scales of order ∼400  Mpc if f≳10−4.

  • 252.
    Prokhorov, Dmitry
    et al.
    Korea Astron & Space Sci Inst, South Korea.
    Antonuccio-Delogu, V.
    Osserv Astrofis Catania, Italy ; Scuola Super Catania, Italy.
    Silk, J.
    Univ Oxford, UK ; Inst Astrophys Paris, France.
    Comptonization of the cosmic microwave background by high energy particles residing in AGN cocoons2010Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 520, artikel-id A106Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. X-ray cavities and extended radio sources ("cocoons") surrounding active galactic nuclei (AGN) have been detected by the Chandra X-ray mission and radio interferometers. A joint analysis of X-ray and radio maps suggests that pressure values of non-thermal radio-emitting particles derived from the radio maps are not sufficient to inflate the X-ray cavities. We propose using the Sunyaev-Zel'dovich (SZ) effect, whose intensity strongly depends on the pressure, to find the hitherto undetected, dynamically-dominant component in the radio cocoons. Aims. Numerical simulations show that plasma with a high temperature (10(9)-10(10) K) is a good candidate for inflating the AGN cocoons. To constrain the population of high energy electrons inside AGN cocoons that is predicted by numerical simulations, we study different methods for maximizing the contribution of such energetic electrons to the SZ effect. Methods. Our calculations of intensity maps of the SZ effect include relativistic corrections and utilize both analytic models and numerical 2D simulations. Results. We demonstrate that the spectral function at a frequency of 217 GHz has an absolute maximum at a temperature higher than 10(9) K, therefore the measurement of the SZ effect at this frequency is a powerful tool for potentially revealing the dynamically-dominant component inside AGN jet-driven radio cocoons. A new method is proposed for excluding the contribution from the low energy, non-relativistic electrons to the SZ effect by means of observations at two frequencies. We show how one may correct for a possible contribution from the kinematic SZ effect. The intensity maps of the SZ effect are calculated for the self-similar Sedov solution, and application of a predicted ring-like structure on the SZ map at a frequency of 217 GHz is proposed to determine the energy released during the active jet stage. The SZ intensity map for an AGN cocoon in a distant elliptical is calculated using a 2D numerical simulation and including relativistic corrections to the SZ effect. We show the intensity spectrum of the SZ effect is flat at high frequencies if gas temperature is as high as k(b)T(e) = 500 keV.

  • 253.
    Prokhorov, Dmitry
    et al.
    Max Planck Inst Astrophys, Germany.
    Churazov, E. M.
    Max Planck Inst Astrophys, Germany ; Space Res Inst IKI, Russia.
    Counting gamma rays in the directions of galaxy clusters2014Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 567, artikel-id A93Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Emission from active galactic nuclei (AGNs) and from neutral pion decay are the two most natural mechanisms that could establish a galaxy cluster as a source of gamma rays in the GeV regime. We revisit this problem by using 52.5 months of Fermi-LAT data above 10 GeV and stacking 55 clusters from the HIFLUCGS sample of the X-ray brightest clusters. The choice of >10 GeV photons is optimal from the point of view of angular resolution, while the sample selection optimizes the chances of detecting signatures of neutral pion decay, arising from hadronic interactions of relativistic protons with an intracluster medium, which scale with the X-ray flux. In the stacked data we detected a signal for the central 0.25 deg circle at the level of 4.3 sigma. Evidence for a spatial extent of the signal is marginal. A subsample of cool-core clusters has a higher count rate of 1.9 +/- 0.3 per cluster compared to the subsample of non-cool core clusters at 1.3 +/- 0.2. Several independent arguments suggest that the contribution of AGNs to the observed signal is substantial, if not dominant. No strong support for the large contribution of pion decay was found. In terms of a limit on the relativistic proton energy density, we derived an upper limit of similar or equal to 2% relative to the gas thermal energy density, provided that the spectrum of relativistic protons is hard (s = 4.1 in dN alpha p(-s) d(3) p). This estimate assumes that relativistic and thermal components are mixed. For softer spectra the upper limits are weaker and equal to 3% for s = 4.2. 4% for s = 4.3, and 6% for s = 4.4.

  • 254.
    Prokhorov, Dmitry
    et al.
    Stanford Univ, USA.
    Colafrancesco, S.
    Univ Witwatersrand, South Africa ; Osserv Astron Roma, Italy.
    The first measurement of temperature standard deviation along the line of sight in galaxy clusters2012Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 424, nr 1, s. L49-L53Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Clusters of galaxies are mainly formed by merging of smaller structures, according to the standard cosmological scenario. If the mass of a substructure is greater than or similar to 10 per cent of that of a galaxy cluster, the temperature distribution of the intracluster medium (ICM) in a merging cluster becomes inhomogeneous. Various methods have been used to derive the two-dimensional projected temperature distribution of the ICM. However, methods for studying temperature distribution along the line of sight through the cluster were absent. In this Letter, we present the first measurement of the temperature standard deviation along the line of sight, using as a reference case the multifrequency Sunyaev-Zel'dovich measurements of the Bullet Cluster. We find that the value of the temperature standard deviation is high and equals to (10.6 +/- 3.8) keV in the Bullet Cluster. This result shows that the temperature distribution in the Bullet Cluster is strongly inhomogeneous along the line of sight and provides a new method for studying galaxy clusters in depth.

  • 255.
    Prokhorov, Dmitry
    et al.
    Stanford University, USA.
    Colafrancesco, S.
    INAF - Osservatorio Astronomico di Roma, Italy.
    Akahori, T.
    Chungnam National University, Republic of Korea.
    Million, E. T.
    University of Alabama, USA.
    Nagataki, S.
    Kyoto University, Japan.
    Yoshikawa, K.
    University of Tsukuba, Japan.
    A high-frequency study of the Sunyaev-Zel'dovich effect morphology in galaxy clusters2011Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 416, nr 1, s. 302-310Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-frequency, high-resolution imaging of the Sunyaev-Zel'dovich (SZ) effect is an important technique to study the complex structures of the atmospheres of merging galaxy clusters. Such observations are sensitive to the details of the electron spectrum. We show that the morphology of the SZ intensity maps in simulated galaxy clusters observed at 345, 600 and 857 GHz are significantly different because of SZ relativistic corrections. These differences can be revealed by high-resolution imaging instruments. We calculate relativistically corrected SZ intensity maps of a simulated, massive, merging galaxy cluster and of the massive, merging clusters 1E0657-558 (the Bullet cluster) and Abell 2219. The morphologies of the SZ intensity maps are remarkably different between 345 and 857 GHz for each merging cluster. We show that high-resolution imaging observations of the SZ intensity maps at these frequencies, obtainable with the LABOCA and HERSCHEL-SPIRE instruments, allow to fully exploit the astrophysical relevance of the predicted SZ morphological effect.

  • 256.
    Prokhorov, Dmitry
    et al.
    Korea Astronomy and Space Science Institute, Republic of Korea ; Stanford University, USA ; Kyoto Univiversity, Japan.
    Colafrancesco, S.
    INAF Osservatorio Astronomico Roma, Italy ; Agenzia Spaziale Italiana, Italy.
    Akahori, T.
    Chungnam National University, Republic of Korea.
    Yoshikawa, K.
    University of Tsukuba, Japan.
    Nagataki, S.
    Kyoto University, Japan.
    Seon, K. -I
    Korea Astronomy and Space Science Institute, Republic of Korea.
    Can electron distribution functions be derived through the Sunyaev-Zel'dovich effect?2011Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 529, artikel-id A39Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. Measurements of the Sunyaev-Zel'dovich (hereafter SZ) effect distortion of the cosmic microwave background provide methods to derive the gas pressure and temperature of galaxy clusters. Here we study the ability of SZ effect observations to derive the electron distribution function (DF) in massive galaxy clusters. Methods. Our calculations of the SZ effect include relativistic corrections considered within the framework of the Wright formalism and use a decomposition technique of electron DFs into Fourier series. Using multi-frequency measurements of the SZ effect, we find the solution of a linear system of equations that is used to derive the Fourier coefficients; we further analyze different frequency samples to decrease uncertainties in Fourier coefficient estimations. Results. We propose a method to derive DFs of electrons using SZ multi-frequency observations of massive galaxy clusters. We found that the best frequency sample to derive an electron DF includes high frequencies nu = 375, 600, 700, 857 GHz. We show that it is possible to distinguish a Juttner DF from a Maxwell-Bolzman DF as well as from a Juttner DF with the second electron population by means of SZ observations for the best frequency sample if the precision of SZ intensity measurements is less than 0.1%. We demonstrate by means of 3D hydrodynamic numerical simulations of a hot merging galaxy cluster that the morphologies of SZ intensity maps are different for frequencies nu = 375, 600, 700, 857 GHz. We stress that measurements of SZ intensities at these frequencies are a promising tool for studying electron distribution functions in galaxy clusters.

  • 257.
    Prokhorov, Dmitry
    et al.
    Max Planck Inst Astrophys, Germany.
    de Jong, S.
    Univ Paris Diderot, France.
    A morphological analysis for searches of possible extended gamma-ray sources associated with dark matter annihilation2014Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 441, nr 3, s. 2200-2207Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose a morphological analysis for searches of extended gamma-ray emission associated with dark matter annihilation. Our approach is based on the likelihood analysis including the spatial templates produced by taking into account the residual count maps in the energy band where the dark matter annihilation spectrum has a prominent spectral feature. The approach is tested on the example of the possible dark matter annihilation signal from the Virgo cluster of galaxies.

  • 258.
    Prokhorov, Dmitry
    et al.
    Korea Astron & Space Sci Inst, South Korea ; Kyoto Univ, Japan.
    Dubois, Y.
    Univ Oxford, UK.
    Nagataki, S.
    Kyoto Univ, Japan.
    An analysis of the temperature structure of galaxy clusters by means of the thermal Sunyaev-Zel'dovich effect2010Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 524, artikel-id A89Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Measurements of the Sunyaev-Zel'dovich (hereafter SZ) effect distortion of the cosmic microwave background provide us with an independent method to derive the gas temperature of galaxy clusters. In merging galaxy clusters the gas distribution is inhomogeneous and, therefore, the method of temperature measuring based on the SZ effect should be more relevant than that based on an X-ray emission analysis. Aims. We study a method for measuring the gas temperature in merging clusters by means of the SZ effect. Methods. Our calculations of intensity maps of the SZ effect include relativistic corrections considered within the framework of the Wright formalism and utilize a cosmological numerical simulation of a merging galaxy cluster evolved with its baryon physics. Results. We found that the gas temperature in merging clusters can be inferred from the ratio of the SZ intensity at a low frequency (128 GHz) to that at a high frequency (369 GHz). This SZ intensity ratio permits us to reveal prominent features of the temperature structure caused by violent merger shock waves. Therefore, measurements of the ratio of the SZ intensities are a promising tool for measuring the gas temperature in merging galaxy clusters.

  • 259.
    Prokhorov, Dmitry
    et al.
    Stanford Univ, USA.
    Dubois, Y.
    Univ Oxford, UK.
    Nagataki, S.
    Kyoto Univ, Japan.
    Akahori, T.
    Chungnam Natl Univ, South Korea.
    Yoshikawa, K.
    Univ Tsukuba, Japan.
    Unveiling the 3D temperature structure of galaxy clusters by means of the thermal Sunyaev-Zel'dovich effect2011Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, nr 3, s. 2505-2512Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Sunyaev-Zel'dovich (hereinafter SZ) effect is a promising tool to derive the gas temperature of galaxy clusters. Approximation of a spherically symmetric gas distribution is usually used to determine the temperature structure of galaxy clusters, but this approximation cannot properly describe merging galaxy clusters. The methods used so far, which do not assume the spherically symmetric distribution, permit us to derive 2D temperature maps of merging galaxy clusters. In this paper, we propose a method to derive the standard temperature deviation and temperature variance along the line of sight, which permits us to analyse the 3D temperature structure of galaxy clusters by means of the thermal SZ effect. We also propose a method to reveal merger shock waves in galaxy clusters by analysing the presence of temperature inhomogeneities along the line of sight.

  • 260.
    Prokhorov, Dmitry
    et al.
    Univ Paris 06, CNRS, France ; Moscow Inst Phys & Technol, Russia.
    Durret, F.
    An approximate theory for substructure propagation in clusters2007Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 474, nr 2, s. 375-384Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. The existence of dark matter can be proved in an astrophysical context by the discovery of a system in which the observed baryons and the inferred dark matter are spatially segregated, such as the bullet cluster (1E0657-558). The full descriptions of the dark matter halo and X-ray gas substructure motions are necessary to forecast the location of the dark halo from X-ray maps, which can be confirmed by the detection of a galaxy concentration or by gravitational lensing. Methods. We present an analytical hydrodynamic model to determine the distance between the X-ray and dark-matter components and the Mach number of the merger shock. Results. An approximate solution is given for the problem of the substructure propagation in merging clusters. A new method to predict the position of a dark matter halo in clusters, where there is a separation between the X-ray gas and the dark halo, is proposed and applied to the clusters 1E0657-558 and Abell 1763.

  • 261.
    Prokhorov, Dmitry
    et al.
    Univ Paris 06, CNRS, France ; Moscow Inst Phys & Technol, Russia.
    Durret, F.
    Univ Paris 06, CNRS, France.
    Dogiel, V.
    PN Lebedev Phys Inst, Russia.
    Colafrancesco, S.
    INAF Osservatorio Astron Roma, Italy.
    An analysis of electron distributions in galaxy clusters by means of the flux ratio of iron lines FeXXV and XXVI2009Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 496, nr 1, s. 25-30Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims The interpretation of hard X-ray emission from galaxy clusters is still ambiguous and different proposed models can be probed using various observational methods. Here we explore a new method based on Fe-line observations. Methods. Spectral-line emissivities have usually been calculated by assuming a Maxwellian electron distribution. In this paper, a generalized approach to calculating the iron-line flux for a modified Maxwellian distribution is considered. Results. We calculated the flux ratio of iron lines for various possible populations of electrons proposed to account for measurements of hard X-ray excess-emission from the clusters A2199 and Coma. We found that the influence of the suprathermal electron population on the flux ratio is more significant in low temperature clusters (as Abell 2199) than in high temperature clusters (as Coma).

  • 262.
    Prokhorov, Dmitry
    et al.
    Stanford Univ, USA.
    Million, E. T.
    Akahori, T.
    Zemcov, M.
    Moraghan, A.
    Nagataki, S.
    Yoshikawa, K.
    Colafrancesco, S.
    Rawle, T. D.
    Egami, E.
    A high-resolution study of the X-ray emission and Sunyaev-Zel'dovich effect in the Bullet cluster (1E 0657-56)2012Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 426, nr 3, s. 2291-2299Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-resolution imaging of the SunyaevZel'dovich (SZ) effect opens new possibilities for testing the presence of various high-energy particle populations in clusters of galaxies. A detailed X-ray analysis of the Bullet cluster (1E 0657-56) with Chandra has revealed the presence of additional X-ray spectral components beyond a simple, single-temperature plasma in its X-ray spectra. X-ray methods alone are insufficient to elucidate the origins of these spectral components. We show that the morphology and magnitude of the SZ effect at high frequencies are critically dependent upon the mechanism by which the additional X-ray spectra are created. We examine the differences between the predicted SZ effect emission maps at 600?GHz assuming the X-ray spectra are composed of thermal gas with a steep power-law index component and also thermal gas with a significant contribution of strongly heated gas. A two-temperature model with a hot (kT ? 3040?keV) second component is the most consistent with existing SZ data at high frequencies. However, significant morphological differences remain. High-angular-resolution SZ intensity maps at high frequencies in combination with deep X-ray data provide a new window into understanding particle energization processes in the hottest, massive merging galaxy clusters.

  • 263.
    Prokhorov, Dmitry
    et al.
    Stanford Univ, USA.
    Moraghan, A.
    Antonuccio-Delogu, V.
    Silk, J.
    Simulating Sunyaev-Zel'dovich intensity maps of giant active galactic nucleus cocoons2012Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 425, nr 3, s. 1753-1762Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform relativistic hydrodynamic simulations of the formation and evolution of active galactic nucleus (AGN) cocoons produced by very light powerful jets. We calculate the intensity maps of the SunyaevZel'dovich (SZ) effect at high frequencies for the simulated AGN cocoons using the relativistically correct Wright formalism. Our fully relativistic calculations demonstrate that the contribution from the high-temperature gas (kbTe ? 100?keV) to the SZ signal from AGN cocoons at high frequencies is stronger than that from the shocked ambient intercluster medium owing to the fact that the relativistic spectral functions peak at these temperature values. We present simulations of the SZ effect from AGN cocoons at various frequencies, and demonstrate that SZ observations at 217 GHz and at higher frequencies, such as 857 GHz, will provide us with knowledge about the dynamically dominant component of AGN cocoons.

  • 264.
    Prokhorov, Dmitry
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Moraghan, Anthony
    Academia Sinica Institute of Astronomy and Astrophysics, Taiwan ; Yonsei University, South Korea.
    A search for pair haloes around active galactic nuclei through a temporal analysis of Fermi-Large Area Telescope data2016Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 457, nr 3, s. 2433-2444Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We develop a method to search for pair haloes around active galactic nuclei (AGN) through a temporal analysis of γ-ray data. The basis of our method is an analysis of the spatial distributions of photons coming from AGN flares and from AGN quiescent states and a further comparison of these two spatial distributions. This method can also be used for a reconstruction of a point spread function (PSF). We found no evidence for a pair halo component through this method by applying it to the Fermi-Large Area Telescope (LAT) data in the energy bands of 4.5–6, 6–10, and >10 GeV and set upper limits on the fraction of photons attributable to a pair halo component. An illustration of how to reconstruct the PSF ofFermi-LAT is given. We demonstrate that the PSF reconstructed by using this method is in good agreement with that which was obtained by using the γ-ray data taken by LAT in the direction of the Crab pulsar and nebula.

  • 265.
    Prokhorov, Dmitry
    et al.
    Korea Astron & Space Sci Inst, South Korea.
    Silk, Joseph
    Univ Oxford, UK ; Inst Astrophys, France.
    CAN THE EXCESS IN THE Fe XXVI Ly gamma LINE FROM THE GALACTIC CENTER PROVIDE EVIDENCE FOR 17 keV STERILE NEUTRINOS?2010Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 725, nr 2, s. L131-L134Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Sterile neutrinos (or right-handed neutrinos) are a plausible warm dark matter candidate. We find that the excess of the intensity in the 8.7 keV line (at the energy of the Fe XXVI Ly gamma line) in the spectrum of the Galactic center observed by Suzaku cannot be explained by standard ionization and recombination processes. We suggest that the origin of this excess is via decays of sterile neutrinos with a mass of 17.4 keV. The estimated value of the mixing angle sin(2)(2 theta) = (4.4 +/- 2.2) x 10(-12) lies in the allowed region of the mixing angle for a dark matter sterile neutrino with a mass of 17-18 keV.

  • 266.
    Prokoph, Heike
    Deutsches Elektronen-Synchrotron (DESY), Germany.
    VERITAS Observations of Relativistic Outflows in Low- and Intermediate-Frequency BL Lac Objects2014Ingår i: International Journal of Modern Physics, Conference Series, ISSN 2010-1945, Vol. 28, artikel-id 1460178Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The majority of blazars detected at very high energies (VHE; E > 100 GeV) are high-frequency-peaked BL Lac objects (HBLs). Low- and intermediate-frequency-peaked BL Lacs (LBLs/IBLs with synchrotron-peak frequencies in the infrared and optical regime) are generally more powerful, more luminous, and have a richer jet environment than HBLs. However, only a handful of these IBL and LBLs have been detected by ground-based gamma-ray telescopes, typically during high-flux states. The VERITAS array has been monitoring five known VHE LBLs/IBLs since 2009: 3C 66A, W Comae, PKS 1424+240, S5 0716+714 and BL Lacertae, with typical exposures of 5-10 hours per year. The results of these long-term observations are presented, including a bright, subhour-scale VHE flare of BL Lacertae in June 2011, the first low-state detections of 3C 66A and W Comae, and the detection and characterization of the IBL B2 1215+30.

  • 267.
    Prokoph, Heike
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Becherini, Yvonne
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Böttcher, M.
    North-West University, South Africa.
    Boisson, C.
    Université Paris Diderot, France.
    Lenain, J.-P.
    Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, France.
    Sushch, I.
    North-West University, South Africa.
    Collaboration, for the H. E. S. S.
    H.E.S.S. discovery of very-high-energy gamma-ray emission of PKS 1440-3892015Ingår i: Proceedings of Science, SISSA , 2015Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Blazars are the most abundant class of known extragalactic very-high-energy (VHE, E>100 GeV) gamma-ray sources. However, one of the biggest difficulties in investigating their VHE emission resides in their limited number, since less than 60 of them are known by now. In this contribution we report on H.E.S.S. observations of the BL Lac object PKS 1440-389. This source has been selected as target for H.E.S.S. based on its high-energy gamma-ray properties measured by Fermi-LAT. The extrapolation of this bright, hard-spectrum gamma-ray blazar into the VHE regime made a detection on a relatively short time scale very likely, despite its uncertain redshift. H.E.S.S. observations were carried out with the 4-telescope array from February to May 2012 and resulted in a clear detection of the source. Contemporaneous multi-wavelength data are used to construct the spectral energy distribution of PKS 1440-389 which can be described by a simple one-zone synchrotron-self Compton model

  • 268.
    Prokoph, Heike
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Collaboration, VERITAS
    Da Vela, P.
    Università di Siena and INFN Pisa, Italy.
    Schultz, C.
    Università di Padova and INFN, Italy.
    Collaboration, MAGIC
    Time-resolved multiwavelength observations of the blazar VER J0521+211 from radio to gamma-ray energies2015Ingår i: Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague (The Netherlands), 2015Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    VER J0521+211 (RGB J0521.8+2112) is one of the brightest and most powerful blazars detected in the TeV gamma-ray regime. It is located at a redshift of z=0.108 and since its discovery in 2009, VER J0521+211 has exhibited an average TeV flux exceeding 0.1 times that of the Crab Nebula, corresponding to an isotropic luminosity of 3×1044 erg s−1. We present data from a comprehensive multiwavelength campaign on this object extending between November 2012 and February 2014, including single-dish radio observations, optical photometry and polarimetry, UV, X-ray, GeV and TeV gamma-ray data (VERITAS, MAGIC). Significant flux variability was observed at all wavelengths, including a long-lasting high state at gamma-ray energies in Fall 2013. Nightly-resolved spectra at X-ray and TeV energies are be presented, and emission mechanisms explaining the observed flux and spectral variability are discussed. 

  • 269.
    Rannot, R. C.
    et al.
    Bhabha Atomic Research Centre, India.
    Chandra, P.
    Bhabha Atomic Research Centre, India.
    Thoudam, Satyendra
    Bhabha Atomic Research Centre, India.
    Yadav, K.
    Bhabha Atomic Research Centre, India.
    Venugopal, K.
    Bhabha Atomic Research Centre, India.
    Bhatt, N.
    Bhabha Atomic Research Centre, India.
    Bhattacharyya, S.
    Bhabha Atomic Research Centre, India.
    Dhar, V. K.
    Bhabha Atomic Research Centre, India.
    Goyal, H. C.
    Bhabha Atomic Research Centre, India.
    Godambe, S.
    Bhabha Atomic Research Centre, India.
    Kaul, R. K.
    Bhabha Atomic Research Centre, India.
    Kothari, M.
    Bhabha Atomic Research Centre, India.
    Kotwal, S.
    Bhabha Atomic Research Centre, India.
    Koul, R.
    Bhabha Atomic Research Centre, India.
    Tickoo, A. K.
    Bhabha Atomic Research Centre, India.
    Sahayanathan, S.
    Bhabha Atomic Research Centre, India.
    Sapru, M. L.
    Bhabha Atomic Research Centre, India.
    Koul, M. K.
    Bhabha Atomic Research Centre, India.
    Sharma, M.
    Bhabha Atomic Research Centre, India.
    TeV gamma-ray Observations of the Blazar Markarian 421 from January to April 2004 with TACTIC Imaging Element2005Ingår i: 29th International Cosmic Ray Conference Pune (2005), 2005, Vol. 4, s. 355-358Konferensbidrag (Refereegranskat)
    Abstract [en]

    We have observed the blazar Markarian 421 (z=0.031 ) with the TACTIC gamma- ray telescope having thresholdenergy of 1.5TeV at Mt. Abu Rajasthan India from January to April 2004. Observations were made intracking mode for a total of 84(on) /16 (off-source) hours with its imaging element equipped with a 349- pixelPMTs camera. During this period, we nd an evidence of a TeV gamma-ray signal with a statistical signi-cance of 6.8. The differential energy spectrum derived in the energy range 2 - 9 TeV is well represented bya simple power law, with a differential spectral index of 2.800.20.

  • 270. Raue, M.
    et al.
    Lenain, J. -P
    Aharonian, F. A.
    Becherini, Yvonne
    Boisson, C.
    Clapson, A. -C
    Costamante, L.
    Gerard, L.
    Medina, C.
    de Naurois, M.
    Punch, M.
    Univ Paris Diderot, APC, AstroParticule & Cosmology, CNRS,IN2P3,CEA,Irfu, Observ Paris,Sorbonne Paris C, 10 Rue Alice Domon & Leonie Duquet, F-75205 Paris 13, France.
    Rieger, F.
    So, H.
    Stawarz, L.
    Zech, A.
    Discovery of VHE gamma-rays from Centaurus A2010Ingår i: Accretion and Ejection in AGN: a Global View / [ed] Maraschi, L; Ghisellini, G; DellaCeca, R; Tavecchio, F, Astronomical Society of the Pacific , 2010, Vol. 427, s. 302-303Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Centaurus A is the closest active radio galaxy. Its proximity enables unique studies of the processes in relativistic jets and near supermassive black holes. Centaurus A has been intensively studied in many wavelength bands and has also been discussed as a possible site for UHECR acceleration. Here, we report the discovery of weak very-high energy gamma-ray emission from Centaurus A with the H.E.S.S. Cherenkov telescope array. The discovery of Centaurus A, together with the detection of M 87, establishes radio galaxies - i.e. AGNs with only weak relativistic beaming - as sources of VHE gamma-rays. The implications of these results on the emission site and on the different models available will be discussed.

  • 271.
    Rossetto, L.
    et al.
    Radboud University, Netherlands.
    Bonardi, A.
    Radboud University, Netherlands.
    Buitink, S.
    Vrije Universiteit Brussel, Belgium.
    Corstanje, A.
    Radboud University, Netherlands.
    Falcke, H.
    Radboud University, Netherlands;NIKHEF, Netherlands;Netherlands Institute of Radio Astronomy (ASTRON), Netherlands.
    Hare, B. M.
    University Groningen, Netherlands.
    Hörandel, J. R.
    Radboud University, Netherlands;NIKHEF, Netherlands.
    Mitra, P.
    Vrije Universiteit Brussel, Belgium.
    Mulrey, K.
    Vrije Universiteit Brussel, Belgium.
    Nelles, A.
    Radboud University, Netherlands;University of California Irvine, USA.
    Olid Gonzalez, A.
    Radboud University, Netherlands.
    Rachen, J. P.
    Radboud University, Netherlands.
    Schellart, P.
    Radboud University, Netherlands;Vrije Universiteit Brussel, Belgium.
    Scholten, O.
    University Groningen, Netherlands;Vrije Universiteit Brussel, Belgium.
    Ter Veen, S.
    Radboud University, Netherlands;Netherlands Institute of Radio Astronomy (ASTRON), Netherlands.
    Thoudam, Satyendra
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE). Radboud University, Netherlands.
    Trinh, T. N. G.
    University Groningen, Netherlands.
    Winchen, T.
    Vrije Universiteit Brussel, Belgium.
    Characterisation of the radio frequency spectrum emitted by high energy air showers with LOFAR2018Ingår i: 35th International Cosmic Ray Conference, ICRC2017, Trieste: Sissa Medialab Srl , 2018, artikel-id 329Konferensbidrag (Refereegranskat)
    Abstract [en]

    The high number density of radio antennas at the LOFAR core in Northern Netherlands allows to detect radio signals emitted by extensive air showers in the energy range 1016 - 1018 e V, and to characterise the geometry of the observed cascade in a detailed way. The radio signal emitted by extensive air showers along their propagation in the atmosphere has been studied in the 30 - 70 MHz frequency range. The study has been conducted on real data and simulated showers. Regarding real data, cosmic ray radio signals detected by LOFAR since 2011 have been analysed. For simulated showers, the CoREAS code, a plug-in of the CORSIKA particle simulation code, has been used. The results show a clear dependence of the frequency spectrum on the distance to the shower axis for both real data and simulations. In particular, the spectrum flatten at a distance around 100 m from the shower axis, where the coherence of the radio signal is maximum. This behaviour could also be used to reconstruct the position of the shower axis at ground. A correlation between the frequency spectrum and the geometrical distance to the depth of the shower maximum Xmax has also been investigated. The final aim of this study is to find a method to improve the inferred information of primary cosmic rays with radio antennas, in view of affirming the radio detection technique as reliable method for the study of extensive air showers. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).

  • 272.
    Rossetto, L.
    et al.
    Radboud Univ Nijmegen, Netherlands.
    Buitink, S.
    Vrije Univ Brussel, Belgium.
    Corstanje, A.
    Radboud Univ Nijmegen, Netherlands.
    Enriquez, J. E.
    Radboud Univ Nijmegen, Netherlands.
    Falcke, H.
    Radboud Univ Nijmegen, Netherlands ; NIKHEF, Netherlands ; Netherlands Inst Radio Astron ASTRON, Netherlands.
    Horandel, J. R.
    Radboud Univ Nijmegen, Netherlands ; NIKHEF, Netherlands.
    Nelles, A.
    Univ Calif Irvine, USA.
    Rachen, J. P.
    Radboud Univ Nijmegen, Netherlands.
    Schellart, P.
    Radboud Univ Nijmegen, Netherlands.
    Scholten, O.
    Univ Groningen, Netherlands ; Vrije Univ Brussel, Belgium.
    ter Veen, S.
    Radboud Univ Nijmegen, Netherlands ; Netherlands Inst Radio Astron ASTRON, Netherlands.
    Thoudam, Satyendra
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Trinh, T. N. G.
    Univ Groningen, Netherlands.
    Measurement of cosmic rays with LOFAR2016Ingår i: XIV INTERNATIONAL CONFERENCE ON TOPICS IN ASTROPARTICLE AND UNDERGROUND PHYSICS (TAUP 2015), PTS 1-7, Institute of Physics Publishing (IOPP), 2016, artikel-id UNSP 052035Konferensbidrag (Refereegranskat)
    Abstract [en]

    The LOw Frequency ARay (LOFAR) is a multipurpose radio -antenna array aimed to detect radio signals in the 10 - 240 MHz frequency range, covering a large surface in Northern Europe with a higher density in the Northern Netherlands. Radio emission in the atmosphere is produced by cosmic -ray induced air showers through the interaction of charged particles with the Earth magnetic field. The detection of radio signals allows to reconstruct several properties of the observed cascade. We review here all important results achieved in the last years. We proved that the radio -signal distribution at ground level is described by a two-dimensional pattern, which is well fitted by a double Gaussian function. The radio -signal arrival time and polarization have been measured, thus providing additional information on the extensive air shower geometry, and on the radio emission processes. We also showed that the radio signal reaches ground in a thin, curved wavefront which is best parametrized by a hyperboloid shape centred around the shower axis. Radio emission has also been studied under thunderstorm conditions and compared to fair weather conditions. Moreover, by using a hybrid reconstruction technique, we performed mass composition measurements in the energy range 10(17) - 10(18) eV.

  • 273.
    Sapru, M. L.
    et al.
    Bhabha Atomic Research Centre, India.
    Tickoo, A. K.
    Bhabha Atomic Research Centre, India.
    Dhar, V. K.
    Bhabha Atomic Research Centre, India.
    Thoudam, Satyendra
    Bhabha Atomic Research Centre, India.
    Rannot, R. C.
    Bhabha Atomic Research Centre, India.
    Koul, R.
    Bhabha Atomic Research Centre, India.
    Threshold energy estimate of the proposed MACE gamma-ray telescope at Hanle2005Ingår i: 29th International Cosmic Ray Conference Pune (2005), 2005, Vol. 5, s. 263-266Konferensbidrag (Refereegranskat)
    Abstract [en]

    The fact that EGRET detected over 250 objects above 100 MeV while only a handful of sources have beendetected above 100 GeV indicates that there is a good chance of detecting a signicant number of these sources(particularly those which do not show a signature of steepening or cut off up to a few GeV), if the thresholdenergy of ground based telescopes is lowered to20 GeV. In the present study, we report the threshold energyestimate of the MACE (Major Atmospheric Cerenkov Experiment) imaging telescope, proposed to be installedin the campus of Indian Astronomical Observatory at Hanle (32.8N, 78.9E, 4200m asl). The results ofthe Monte Carlo simulations carried out with CORSIKA code, suggest that using a pixel threshold of4peand a nearest neighbour quadruplet trigger,-ray energy threshold of15GeV is achievable by the MACEtelescope. Details of the simulation work for estimating the threshold energy along with results obtained arepresented in this paper.

  • 274.
    Schellart, P.
    et al.
    Radboud University Nijmegen, The Netherlands.
    Buitink, S.
    Radboud University Nijmegen, The Netherlands ; University Groningen, The Netherlands.
    Corstanje, A.
    Radboud University Nijmegen, The Netherlands.
    Enriquez, J. E.
    Radboud University Nijmegen, The Netherlands.
    Falcke, H.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands ; Netherlands Institute for Radio Astronomy (ASTRON), The Netherlands ; Max-Planck-Institut für Radioastronomie, Germany.
    Frieswijk, W.
    Netherlands Institute for Radio Astronomy (ASTRON), The Netherlands.
    Hörandel, J. R.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Krause, M.
    Radboud University Nijmegen, The Netherlands.
    Nelles, A.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Scholten, O.
    University Groningen, The Netherlands.
    ter Veen, S.
    Radboud University Nijmegen, The Netherlands.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    van den Akker, M.
    Radboud University Nijmegen, The Netherlands.
    Recent results from cosmic-ray measurements with LOFAR2014Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 742, s. 115-118Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    LOFAR, the Low Frequency Array, is currently the world's largest distributed radio telescope observing at frequencies below 240 MHz. LOFAR is measuring cosmic-ray induced air-showers since June 2011 and has collected several hundreds of events with hundreds of antennas per individual event. We present measurements of the radio signal strength as well as high-precision measurements of wavefront curvature and polarization. These will enable us to disentangle the different emission mechanisms at play, such as geomagnetic radiation, charge excess, and Askaryan or Cherenkov effects, leading to a full understanding of the air-shower radio emission. Furthermore we give a first example on how the full complexity of the signal enables radio measurements to be used to study primary particle composition.

  • 275.
    Schellart, P.
    et al.
    Radboud University Nijmegen, The Netherlands.
    Buitink, S.
    Radboud University Nijmegen, The Netherlands.
    Corstanje, A.
    Radboud University Nijmegen, The Netherlands.
    Enriquez, J. E.
    Radboud University Nijmegen, The Netherlands.
    Falcke, H.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands ; Netherlands Institute for Radio Astronomy (ASTRON), The Netherlands ; Max Planck Institute for Radio Astronomy, Germany.
    Hörandel, J. R.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Krause, M.
    Radboud University Nijmegen, The Netherlands ; Deutsches Elektronen-Synchrotron (DESY), Germany.
    Nelles, A.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Rachen, J. P.
    Radboud University Nijmegen, The Netherlands.
    Scholten, O.
    University of Groningen, The Netherlands.
    ter Veen, S.
    Radboud University Nijmegen, The Netherlands.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Trinh, T. N. G.
    University of Groningen, The Netherlands.
    Polarized radio emission from extensive air showers measured with LOFAR2014Ingår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 10Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present LOFAR measurements of radio emission from extensive air showers. We find that this emission is strongly polarized, with a median degree of polarization of nearly

    99%, and that the angle between the polarization direction of the electric field and the Lorentz force acting on the particles, depends on the observer location in the shower plane. This can be understood as a superposition of the radially polarized charge-excess emission mechanism, first proposed by Askaryan and the geomagnetic emission mechanism proposed by Kahn and Lerche. We calculate the relative strengths of both contributions, as quantified by the charge-excess fraction, for 163 individual air showers. We find that the measured charge-excess fraction is higher for air showers arriving from closer to the zenith. Furthermore, the measured charge-excess fraction also increases with increasing observer distance from the air shower symmetry axis. The measured values range from (3.3± 1.0)% for very inclined air showers at 25 m to (20.3± 1.3)% for almost vertical showers at 225 m. Both dependencies are in qualitative agreement with theoretical predictions.

  • 276. Schellart, P.
    et al.
    Nelles, A.
    Buitink, S.
    Corstanje, A.
    Enriquez, J. E.
    Falcke, H.
    Frieswijk, W.
    Hörandel, J. R.
    Horneffer, A.
    James, C. W.
    Krause, M.
    Mevius, M.
    Scholten, O.
    ter Veen, S.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    van den Akker, M.
    Alexov, A.
    Anderson, J.
    Avruch, I. M.
    Bähren, L.
    Beck, R.
    Bell, M. E.
    Bennema, P.
    Bentum, M. J.
    Bernardi, G.
    Best, P.
    Bregman, J.
    Breitling, F.
    Brentjens, M.
    Broderick, J.
    Brüggen, M.
    Ciardi, B.
    Coolen, A.
    de Gasperin, F.
    de Geus, E.
    de Jong, A.
    de Vos, M.
    Duscha, S.
    Eislöffel, J.
    Fallows, R. A.
    Ferrari, C.
    Garrett, M. A.
    Grießmeier, J.
    Grit, T.
    Hamaker, J. P.
    Hassall, T. E.
    Heald, G.
    Hessels, J. W. T.
    Hoeft, M.
    Holties, H. A.
    Iacobelli, M.
    Juette, E.
    Karastergiou, A.
    Klijn, W.
    Kohler, J.
    Kondratiev, V. I.
    Kramer, M.
    Kuniyoshi, M.
    Kuper, G.
    Maat, P.
    Macario, G.
    Mann, G.
    Markoff, S.
    McKay-Bukowski, D.
    McKean, J. P.
    Miller-Jones, J. C. A.
    Mol, J. D.
    Mulcahy, D. D.
    Munk, H.
    Nijboer, R.
    Norden, M. J.
    Orru, E.
    Overeem, R.
    Paas, H.
    Pandey-Pommier, M.
    Pizzo, R.
    Polatidis, A. G.
    Renting, A.
    Romein, J. W.
    Röttgering, H.
    Schoenmakers, A.
    Schwarz, D.
    Sluman, J.
    Smirnov, O.
    Sobey, C.
    Stappers, B. W.
    Steinmetz, M.
    Swinbank, J.
    Tang, Y.
    Tasse, C.
    Toribio, C.
    van Leeuwen, J.
    van Nieuwpoort, R.
    van Weeren, R. J.
    Vermaas, N.
    Vermeulen, R.
    Vocks, C.
    Vogt, C.
    Wijers, R. A. M. J.
    Wijnholds, S. J.
    Wise, M. W.
    Wucknitz, O.
    Yatawatta, S.
    Zarka, P.
    Zensus, A.
    Detecting cosmic rays with the LOFAR radio telescope2013Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 560, s. 1-14, artikel-id A98Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The low frequency array (LOFAR), is the first radio telescope designed with the capability to measure radio emission from cosmic-ray induced air showers in parallel with interferometric observations. In the first ~2 years of observing, 405 cosmic-ray events in the energy range of 1016−1018 eV have been detected in the band from 30−80 MHz. Each of these air showers is registered with up to ~1000 independent antennas resulting in measurements of the radio emission with unprecedented detail. This article describes the dataset, as well as the analysis pipeline, and serves as a reference for future papers based on these data. All steps necessary to achieve a full reconstruction of the electric field at every antenna position are explained, including removal of radio frequency interference, correcting for the antenna response and identification of the pulsed signal.

  • 277. Schellart, P.
    et al.
    Trinh, T. N. G.
    Buitink, S.
    Corstanje, A.
    Enriquez, J. E.
    Falcke, H.
    Hörandel, J. R.
    Nelles, A.
    Rachen, J. P.
    Rossetto, L.
    Scholten, O.
    ter Veen, S.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Ebert, U.
    Koehn, C.
    Rutjes, C.
    Alexov, A.
    Anderson, J. M.
    Avruch, I. M.
    Bentum, M. J.
    Bernardi, G.
    Best, P.
    Bonafede, A.
    Breitling, F.
    Broderick, J. W.
    Brüggen, M.
    Butcher, H. R.
    Ciardi, B.
    de Geus, E.
    de Vos, M.
    Duscha, S.
    Eislöffel, J.
    Fallows, R. A.
    Frieswijk, W.
    Garrett, M. A.
    Grießmeier, J.
    Gunst, A. W.
    Heald, G.
    Hessels, J. W. T.
    Hoeft, M.
    Holties, H. A.
    Juette, E.
    Kondratiev, V. I.
    Kuniyoshi, M.
    Kuper, G.
    Mann, G.
    McFadden, R.
    McKay-Bukowski, D.
    McKean, J. P.
    Mevius, M.
    Moldon, J.
    Norden, M. J.
    Orru, E.
    Paas, H.
    Pandey-Pommier, M.
    Pizzo, R.
    Polatidis, A. G.
    Reich, W.
    Röttgering, H.
    Scaife, A. M. M.
    Schwarz, D. J.
    Serylak, M.
    Smirnov, O.
    Steinmetz, M.
    Swinbank, J.
    Tagger, M.
    Tasse, C.
    Toribio, M. C.
    van Weeren, R. J.
    Vermeulen, R.
    Vocks, C.
    Wise, M. W.
    Wucknitz, O.
    Zarka, P.
    Probing Atmospheric Electric Fields in Thunderstorms through Radio Emission from Cosmic-Ray-Induced Air Showers2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, nr 16, s. 1-5, artikel-id 165001Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present measurements of radio emission from cosmic ray air showers that took place during thunderstorms. The intensity and polarization patterns of these air showers are radically different from those measured during fair-weather conditions. With the use of a simple two-layer model for the atmospheric electric field, these patterns can be well reproduced by state-of-the-art simulation codes. This in turn provides a novel way to study atmospheric electric fields.

  • 278.
    Scholten, Olaf
    et al.
    Univ Groningen, Netherlands ; Vrije Univ Brussel, Belgium.
    Bonardi, Antonio
    Radboud Univ Nijmegen, Netherlands.
    Buitink, Stijn
    Vrije Univ Brussel, Belgium.
    Corstanje, Arthur
    Radboud Univ Nijmegen, Netherlands.
    Ebert, Ute
    Ctr Wiskunde & Informat, Netherlands ; Eindhoven Univ Technol, Netherlands.
    Falcke, Heino
    Netherlands Inst Radio Astron ASTRON, Netherlands ; Radboud Univ Nijmegen, Netherlands.
    Horandel, Jorg
    Radboud Univ Nijmegen, Netherlands.
    Mitra, Pragati
    Vrije Univ Brussel, Belgium.
    Mulrey, Katharine
    Vrije Univ Brussel, Belgium.
    Nelles, Anna
    Univ Calif Irvine, USA.
    Rachen, Jorg
    Radboud Univ Nijmegen, Netherlands.
    Rossetto, Laura
    Radboud Univ Nijmegen, Netherlands.
    Rutjes, Casper
    Ctr Wiskunde & Informat, Amsterdam, Netherlands..
    Schellart, Pim
    Princeton Univ, USA.
    Thoudam, Satyendra
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Trinh, Gia
    Univ Groningen, Netherlands.
    ter Veen, Sander
    Univ Groningen, Netherlands.
    Winchen, Tobias
    Vrije Univ Brussel, Belgium.
    Precision study of radio emission from air showers at LOFAR2017Ingår i: RICAP16, 6TH ROMA INTERNATIONAL CONFERENCE ON ASTROPARTICLE PHYSICS / [ed] Morselli, A Capone, A Fernandez, GR, 2017, artikel-id UNSP 02012Konferensbidrag (Refereegranskat)
    Abstract [en]

    Radio detection as well as modeling of cosmic rays has made enormous progress in the past years. We show this by using the subtle circular polarization of the radio pulse from air showers measured in fair weather conditions and the intensity of radio emission from an air shower under thunderstorm conditions.

  • 279.
    Scholten, Olaf
    et al.
    University of Groningen, The Nederlands;Vrije Universiteit Brussel, Belgium.
    Buitink, Stijn
    Vrije Universiteit Brussel, Belgium.
    Dina, Roxanna
    University of Groningen, The Netherlands.
    Dorosti Hasankiadeh, Qader
    University of Groningen, The Netherlands.
    Frieswijk, Wilfred
    Netherlands Institute for Radio Astronomy (ASTRON), The Netherlands .
    Hendriks, Freddie
    University of Groningen, The Netherlands .
    Muller, Jeroen
    University of Groningen, The Netherlands .
    Sardjan, Danny
    University of Groningen, The Netherlands .
    Trinh, Gia
    University of Groningen, The Netherlands .
    Bonardi, Antonio
    Radboud University, Netherlands.
    Corstanje, Arthur
    Radboud University, Netherlands.
    Ebert, Ute
    CWI, Centrum Wiskunde & Informatica, Netherlands.
    Falcke, Heino
    Netherlands Institute for Radio Astronomy (ASTRON), Netherlands;Radboud University,The Netherlands.
    Hörandel, Jörg
    Radboud University, The Netherlands.
    Mitra, Pragati
    Vrije Universiteit Brussel, Belgium.
    Mulrey, Katharine
    Vrije Universiteit Brussel, Belgium.
    Nelles, Anna
    University of California, USA.
    Rachen, Jörg
    Radboud University, The Netherlands.
    Rossetto, Laura
    Radboud University, The Netherlands.
    Rutjes, Casper
    CWI, Centrum Wiskunde & Informatica, Amsterdam.
    Schellart, Pim
    Princeton University, USA.
    Thoudam, Satyendra
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Ter Veen, Sander
    Netherlands Institute for Radio Astronomy (ASTRON), The Netherlands.
    Winchen, Tobias
    Vrije Universiteit Brussel, Belgium.
    Lightning Imaging with LOFAR2017Ingår i: 7th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2016), EDP Sciences, 2017, Vol. 135, artikel-id 03003Konferensbidrag (Refereegranskat)
    Abstract [en]

    We show that LOFAR can be used as a lightning mapping array with a resolution that is orders of magnitude better than existing arrays. In addition the polarization of the radiation can be used to track the direction of the stepping discharges. © 2017 The Authors, published by EDP Sciences.

  • 280. Shulevski, A.
    et al.
    Morganti, R.
    Barthel, P. D.
    Murgia, M.
    van Weeren, R. J.
    White, G. J.
    Brüggen, M.
    Kunert-Bajraszewska, M.
    Jamrozy, M.
    Best, P. N.
    Röttgering, H. J. A.
    Chyzy, K. T.
    de Gasperin, F.
    Bîrzan, L.
    Brunetti, G.
    Brienza, M.
    Rafferty, D. A.
    Anderson, J.
    Beck, R.
    Deller, A.
    Zarka, P.
    Schwarz, D.
    Mahony, E.
    Orrú, E.
    Bell, M. E.
    Bentum, M. J.
    Bernardi, G.
    Bonafede, A.
    Breitling, F.
    Broderick, J. W.
    Butcher, H. R.
    Carbone, D.
    Ciardi, B.
    de Geus, E.
    Duscha, S.
    Eislöffel, J.
    Engels, D.
    Falcke, H.
    Fallows, R. A.
    Fender, R.
    Ferrari, C.
    Frieswijk, W.
    Garrett, M. A.
    Grießmeier, J.
    Gunst, A. W.
    Heald, G.
    Hoeft, M.
    Hörandel, J.
    Horneffer, A.
    van der Horst, A. J.
    Intema, H.
    Juette, E.
    Karastergiou, A.
    Kondratiev, V. I.
    Kramer, M.
    Kuniyoshi, M.
    Kuper, G.
    Maat, P.
    Mann, G.
    McFadden, R.
    McKay-Bukowski, D.
    McKean, J. P.
    Meulman, H.
    Mulcahy, D. D.
    Munk, H.
    Norden, M. J.
    Paas, H.
    Pandey-Pommier, M.
    Pizzo, R.
    Polatidis, A. G.
    Reich, W.
    Rowlinson, A.
    Scaife, A. M. M.
    Serylak, M.
    Sluman, J.
    Smirnov, O.
    Steinmetz, M.
    Swinbank, J.
    Tagger, M.
    Tang, Y.
    Tasse, C.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Toribio, M. C.
    Vermeulen, R.
    Vocks, C.
    Wijers, R. A. M. J.
    Wise, M. W.
    Wucknitz, O.
    The peculiar radio galaxy 4C 35.06: a case for recurrent AGN activity?2015Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 579, s. 1-10, artikel-id A27Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using observations obtained with the LOw Fequency ARray (LOFAR), the Westerbork Synthesis Radio Telescope (WSRT) and archival Very Large Array (VLA) data, we have traced the radio emission to large scales in the complex source 4C 35.06 located in the core of the galaxy cluster Abell 407. At higher spatial resolution (~ 4″), the source was known to have two inner radio lobes spanning 31 kpc and a diffuse, low-brightness extension running parallel to them, offset by about 11 kpc (in projection). At 62 MHz, we detect the radio emission of this structure extending out to 210 kpc. At 1.4 GHz and intermediate spatial resolution (~ 30″), the structure appears to have a helical morphology. We have derived the characteristics of the radio spectral index across the source. We show that the source morphology is most likely the result of at least two episodes of AGN activity separated by a dormant period of around 35 Myr. The outermost regions of radio emission have a steep spectral index (α< − 1), indicative of old plasma. We connect the spectral index properties of the resolved source structure with the integrated fluxdensity spectral index of 4C 35.06 and suggest an explanation for its unusual integrated flux density spectral shape (a moderately steep power law with no discernible spectral break), possibly providing a proxy for future studies of more distant radio sources through inferring their detailed spectral index properties and activity history from their integrated spectral indices. The AGN is hosted by one of the galaxies located in the cluster core of Abell 407. We propose that it is intermittently active as it moves in the dense environment in the cluster core. In this scenario, the AGN turned on sometime in the past, and has produced the helical pattern of emission, possibly a sign of jet precession/merger during that episode of activity. Using LOFAR, we can trace the relic plasma from that episode of activity out to greater distances from the core than ever before. Using the the WSRT, we detect H I in absorption against the center of the radio source. The absorption profile is relatively broad (FWHM of 288 kms-1), similar to what is found in other clusters. The derived column density is NHI ~ 4 × 1020 cm-2 for a Tspin = 100 K. This detection supports the connection – already suggested for other restarted radio sources – between the presence of cold gas and restarting activity. The cold gas appears to be dominated by a blue-shifted component although the broad H I profile could also include gas with different kinematics. Understanding the duty cycle of the radio emission as well as the triggering mechanism for starting (or restarting) the radio-loud activity can provide important constraints to quantify the impact of AGN feedback on galaxy evolution. The study of these mechanisms at low frequencies using morphological and spectral information promises to bring new important insights in this field.

  • 281. Sobey, C.
    et al.
    Young, N. J.
    Hessels, J. W. T.
    Weltevrede, P.
    Noutsos, A.
    Stappers, B. W.
    Kramer, M.
    Bassa, C.
    Lyne, A. G.
    Kondratiev, V. I.
    Hassall, T. E.
    Keane, E. F.
    Bilous, A. V.
    Breton, R. P.
    Grießmeier, J. -M
    Karastergiou, A.
    Pilia, M.
    Serylak, M.
    Veen, S. t.
    van Leeuwen, J.
    Alexov, A.
    Anderson, J.
    Asgekar, A.
    Avruch, I. M.
    Bell, M. E.
    Bentum, M. J.
    Bernardi, G.
    Best, P.
    Bîrzan, L.
    Bonafede, A.
    Breitling, F.
    Broderick, J.
    Brüggen, M.
    Corstanje, A.
    Carbone, D.
    de Geus, E.
    de Vos, M.
    van Duin, A.
    Duscha, S.
    Eislöffel, J.
    Falcke, H.
    Fallows, R. A.
    Fender, R.
    Ferrari, C.
    Frieswijk, W.
    Garrett, M. A.
    Gunst, A. W.
    Hamaker, J. P.
    Heald, G.
    Hoeft, M.
    Hörandel, J.
    Jütte, E.
    Kuper, G.
    Maat, P.
    Mann, G.
    Markoff, S.
    McFadden, R.
    McKay-Bukowski, D.
    McKean, J. P.
    Mulcahy, D. D.
    Munk, H.
    Nelles, A.
    Norden, M. J.
    Orrù, E.
    Paas, H.
    Pandey-Pommier, M.
    Pandey, V. N.
    Pietka, G.
    Pizzo, R.
    Polatidis, A. G.
    Rafferty, D.
    Renting, A.
    Röttgering, H.
    Rowlinson, A.
    Scaife, A. M. M.
    Schwarz, D.
    Sluman, J.
    Smirnov, O.
    Steinmetz, M.
    Stewart, A.
    Swinbank, J.
    Tagger, M.
    Tang, Y.
    Tasse, C.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Toribio, C.
    Vermeulen, R.
    Vocks, C.
    van Weeren, R. J.
    Wijers, R. A. M. J.
    Wise, M. W.
    Wucknitz, O.
    Yatawatta, S.
    Zarka, P.
    LOFAR discovery of a quiet emission mode in PSR B0823+262015Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 451, s. 2493-2506Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    PSR B0823+26, a 0.53-s radio pulsar, displays a host of emission phenomena over time-scales of seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like PSR B0823+26 provides further insight into the relationship between these various emission phenomena and what they might teach us about pulsar magnetospheres. Here we report on the LOFAR (Low-Frequency Array) discovery that PSR B0823+26 has a weak and sporadically emitting ‘quiet’ (Q) emission mode that is over 100 times weaker (on average) and has a nulling fraction forty-times greater than that of the more regularly-emitting ‘bright’ (B) mode. Previously, the pulsar has been undetected in the Q mode, and was assumed to be nulling continuously. PSR B0823+26 shows a further decrease in average flux just before the transition into the B mode, and perhaps truly turns off completely at these times. Furthermore, simultaneous observations taken with the LOFAR, Westerbork, Lovell, and Effelsberg telescopes between 110 MHz and 2.7 GHz demonstrate that the transition between the Q mode and B mode occurs within one single rotation of the neutron star, and that it is concurrent across the range of frequencies observed.

  • 282. Stamatescu, V.
    et al.
    Becherini, Yvonne
    APC Paris.
    Bernloehr, K.
    Carmona, E.
    Colin, P.
    Farnier, C.
    Gerard, L.
    Hinton, J. A.
    Khelifi, B.
    Komin, N.
    Lamanna, G.
    Lenain, J. -P
    Maier, G.
    Moralejo, A.
    Naumann, C. L.
    Parsons, D.
    Di Pierro, F.
    Prokoph, Heike
    Vorobiov, S.
    Towards an optimized design for the Cherenkov Telescope Array2012Ingår i: HIGH ENERGY GAMMA-RAY ASTRONOMY, American Institute of Physics (AIP), 2012, s. 758-761Konferensbidrag (Refereegranskat)
    Abstract [en]

    The Cherenkov Telescope Array (CTA) is a future instrument for very-high-energy (VHE) gamma-ray astronomy that is expected to deliver an order of magnitude improvement in sensitivity over existing instruments. In order to meet the physics goals of CTA in a cost-effective way, Monte Carlo simulations of the telescope array are used in its design. Specifically, we simulate large arrays comprising numerous large-size, medium-size and small-size telescopes whose configuration parameters are chosen based on current technical design studies and understanding of the costs involved. Subset candidate arrays with various layout configurations are then selected and evaluated in terms of key performance parameters, such as the sensitivity. This is carried out using a number of data analysis methods, some of which were developed within the field and extended to CTA, while others were developed specifically for this purpose. We outline some key results from recent studies that illustrate our approach to the optimization of the CTA design.

  • 283.
    Stewart, A. J.
    et al.
    University of Oxford, UK ; University of Southampton, UK.
    Fender, R. P.
    University of Oxford, UK ; University of Southampton, UK.
    Broderick, J. W.
    University of Oxford, UK ; University of Southampton, UK ; ASTRON, The Netherlands.
    Hassall, T. E.
    University of Oxford, UK ; University of Southampton, UK.
    Muñoz-Darias, T.
    University of Oxford, UK ; University of Southampton, UK ; Instituto de Astrofísica de Canarias, Spain ; Universidad de La Laguna, Spain.
    Rowlinson, A.
    ASTRON, The Netherlands ; Anton Pannekoek Institute for Astronomy, The Netherlands.
    Swinbank, J. D.
    Anton Pannekoek Institute for Astronomy, The Netherlands ; Princeton University, USA.
    Staley, T. D.
    University of Oxford, UK ; University of Southampton, UK.
    Molenaar, G. J.
    Anton Pannekoek Institute for Astronomy, The Netherlands ; Rhodes University, South Africa.
    Scheers, B.
    Anton Pannekoek Institute for Astronomy, The Netherlands ; Centrum Wiskunde & Informatica, The Netherlands.
    Grobler, T. L.
    Pietka, M.
    Heald, G.
    McKean, J. P.
    Bell, M. E.
    Bonafede, A.
    Breton, R. P.
    Carbone, D.
    Cendes, Y.
    Clarke, A. O.
    Corbel, S.
    de Gasperin, F.
    Eislöffel, J.
    Falcke, H.
    Ferrari, C.
    Grießmeier, J. -M
    Hardcastle, M. J.
    Heesen, V.
    Hessels, J. W. T.
    Horneffer, A.
    Iacobelli, M.
    Jonker, P.
    Karastergiou, A.
    Kokotanekov, G.
    Kondratiev, V. I.
    Kuniyoshi, M.
    Law, C. J.
    van Leeuwen, J.
    Markoff, S.
    Miller-Jones, J. C. A.
    Mulcahy, D.
    Orru, E.
    Pandey-Pommier, M.
    Pratley, L.
    Rol, E.
    Röttgering, H. J. A.
    Scaife, A. M. M.
    Shulevski, A.
    Sobey, C. A.
    Stappers, B. W.
    Tasse, C.
    van der Horst, A. J.
    van Velzen, S.
    van Weeren, R. J.
    Wijers, R. A. M. J.
    Wijnands, R.
    Wise, M.
    Zarka, P.
    Alexov, A.
    Anderson, J.
    Asgekar, A.
    Avruch, I. M.
    Bentum, M. J.
    Bernardi, G.
    Best, P.
    Breitling, F.
    Brüggen, M.
    Butcher, H. R.
    Ciardi, B.
    Conway, J. E.
    Corstanje, A.
    de Geus, E.
    Deller, A.
    Duscha, S.
    Frieswijk, W.
    Garrett, M. A.
    Gunst, A. W.
    van Haarlem, M. P.
    Hoeft, M.
    Hörandel, J.
    Juette, E.
    Kuper, G.
    Loose, M.
    Maat, P.
    McFadden, R.
    McKay-Bukowski, D.
    Moldon, J.
    Munk, H.
    Norden, M. J.
    Paas, H.
    Polatidis, A. G.
    Schwarz, D.
    Sluman, J.
    Smirnov, O.
    Steinmetz, M.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Toribio, M. C.
    Vermeulen, R.
    Vocks, C.
    Wijnholds, S. J.
    Wucknitz, O.
    Yatawatta, S.
    LOFAR MSSS: detection of a low-frequency radio transient in 400 h of monitoring of the North Celestial Pole2016Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, nr 3, s. 2321-2342Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present the results of a four-month campaign searching for low-frequency radio transients near the North Celestial Pole with the Low-Frequency Array (LOFAR), as part of the Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April and comprised 2149 11-min snapshots, each covering 175 deg2. We have found one convincing candidate astrophysical transient, with a duration of a few minutes and a flux density at 60 MHz of 15–25 Jy. The transient does not repeat and has no obvious optical or high-energy counterpart, as a result of which its nature is unclear. The detection of this event implies a transient rate at 60 MHz of 3.9&#x2212;3.7+14.7&#x00D7;10&#x2212;4" style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">3.9+14.7−3.7×10−4 d−1 deg−2, and a transient surface density of 1.5 × 10−5 deg−2, at a 7.9-Jy limiting flux density and ∼10-min time-scale. The campaign data were also searched for transients at a range of other time-scales, from 0.5 to 297 min, which allowed us to place a range of limits on transient rates at 60 MHz as a function of observation duration.

  • 284.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    A possible origin of gamma rays from the Fermi Bubbles2014Ingår i: Nuclear physics B, Proceedings supplements, ISSN 0920-5632, E-ISSN 1873-3832, Vol. 256-257, s. 125-130Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    One of the most exciting discoveries of recent years is a pair of gigantic gamma-ray emission regions, the so-called Fermi bubbles, above and below the Galactic center. The bubbles, discovered by the Fermi space telescope, extend up to ∼50°∼50° in Galactic latitude and are ∼40°∼40° wide in Galactic longitude. The gamma-ray emission is also found to correlate with radio, microwave and X-rays emission. The origin of the bubbles and the associated non-thermal emissions are still not clearly understood. Possible explanations for the non-thermal emission include cosmic-ray injection from the Galactic center by high speed Galactic winds/jets, acceleration by multiple shocks or plasma turbulence present inside the bubbles, and acceleration by strong shock waves associated with the expansion of the bubbles. In this paper, I will discuss the possibility that the gamma-ray emission is produced by the injection of Galactic cosmic-rays mainly protons during their diffusive propagation through the Galaxy. The protons interact with the bubble plasma producing π°π°-decay gamma rays, while at the same time, radio and microwave synchrotron emissions are produced by the secondary electrons/positrons resulting from the π±π± decays.

  • 285.
    Thoudam, Satyendra
    Bhabha Atomic Research Centre, India.
    Cosmic ray proton spectrum below 100 TeV in the local region2006Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 370, nr 1, s. 263-272Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The propagation of cosmic ray (CR) protons in the Galaxy is discussed under the framework of a three-dimensional convection–diffusion model. Starting with the assumption of a uniform and continuous distribution of CR sources injecting CRs continuously in the Galaxy and by invoking a supernova explosion at various distances from the Earth, it is found that only those sources located within a distance of ∼1.5 kpc can produce appreciable temporal fluctuations in the CR proton flux observed near the Earth. So, the construction of the local CR proton spectrum is discussed by separating the contributions of the distant sources from that of the nearby sources. The contribution from the distant sources is treated in the framework of a continuous source distribution model in both space and time, but that of the nearby sources in a discrete space–time source model. The study predicts the presence of at least one old nearby source with a characteristic age of ∼105 yr located at a distance of ∼0.1 kpc to explain the observed proton flux below ∼100 GeV.

  • 286. Thoudam, Satyendra
    Diffuse gamma-ray emission of the galactic disk and Galactic Cosmic-Ray spectra2005Ingår i: International Cosmic Ray Conference, Pune, 2005, 2005, Vol. 4Konferensbidrag (Refereegranskat)
  • 287.
    Thoudam, Satyendra
    Bhabha Atomic Research Centre, India.
    Effect of nearby supernova remnants on local cosmic rays2007Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 378, nr 1, s. 48-54Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study in detail the effect of different particle release times from sources on the cosmic ray (CR) spectrum below 1015 eV in the Galaxy. We discuss different possible forms of particle injection such as burst-like injection, continuous injection for a finite time, injection from a stationary source and energy-dependent injection. When applied to the nearby known supernova remnants, we find that the observed CR anisotropy data favour the burst-like particle injection model for the CR diffusion coefficient D(E) ∝Ea with a= 0.3 –0.6 in the local region. In this study we have also found that the contribution of the sources G114.3+0.3 and Monogem dominate if the observed anisotropy is a result of the effect of the nearby sources. Further study shows that we should not neglect the contribution of the undetected old sources to the local CR anisotropy.

  • 288.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Fermi Bubble γ-Rays as a Result of Diffusive Injection of Galactic Cosmic Rays2013Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 778, nr 1, s. 1-5, artikel-id L20Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recently, the Fermi Space Telescope discovered two large γ-ray emission regions, the so-called Fermi bubbles, that extend up to ~50° above and below the Galactic center (GC). The γ-ray emission from the bubbles is found to follow a hard spectrum with no significant spatial variation in intensity and spectral shape. The origin of the emission is still not clearly understood. Suggested explanations include the injection of cosmic-ray (CR) nuclei from the GC by high-speed Galactic winds, electron acceleration by multiple shocks, and stochastic electron acceleration inside the bubbles. In this Letter, it is proposed that the γ-rays may be the result of diffusive injection of Galactic CR protons during their propagation through the Galaxy. Considering that the bubbles are slowly expanding, and CRs undergo much slower diffusion inside the bubbles than in the average Galaxy and at the same time suffer losses due to adiabatic expansion and inelastic collisions with the bubble plasma, this model can explain the observed intensity profile, the emission spectrum and the measured luminosity without invoking any additional particle production processes, unlike other existing models.

  • 289.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Gamma rays from Fermi bubbles as due to diffusive injection of Galactic cosmic rays2014Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 742, s. 224-227Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent detailed analysis of the Fermi-LAT data has discovered two giant γ-rayγ-ray emission regions, the so-called Fermi bubbles, extending up to ~50° in Galactic latitude above and below the Galactic center with a width of ~40° in longitude. The origin of the γ-rayγ-ray emission is not clearly understood. Here, we discuss the possibility that the γ-raysγ-rays can be the result of diffusive injection of Galactic cosmic-ray protons during their propagation through the Galaxy. In the model, we consider that the bubbles are slowly expanding, and cosmic rays undergo much slower diffusion inside the bubbles than in the averaged Galaxy. Moreover, we consider that cosmic rays inside the bubbles suffer losses from adiabatic expansion, and also from inelastic collisions with the bubble plasma producing pion-decay γ rays. We show that this simple model can explain some of the important properties of Fermi bubbles such as the measured γ-rayγ-ray intensity profile, the energy spectrum and the measured luminosity.

  • 290.
    Thoudam, Satyendra
    Bhabha Atomic Research Centre, India.
    High energy diffuse gamma-ray emission of the galactic disk and galactic cosmic-ray spectra2006Ingår i: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 25, nr 5, s. 328-341Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Observations of diffuse Galactic γ-ray spectrum by the EGRET instrument reveal an excess above ∼1 GeV over the expected γ-ray spectrum calculated under the assumption that the locally observed cosmic-ray (CR) spectra represent the galactic CR spectra. Assuming that Galactic CRs of energy below ∼100 TeV are accelerated by supernova remnant (SNR) shock waves and that the shock compression ratio is SNR age dependent, the average source injection spectra from an ensemble of SNRs is calculated both in the inner (330° < l < 30°) and outer (30° < l < 330°) regions of the galaxy. The calculation considers the SNR age distribution in the galaxy. Injecting these spectra in the galaxy and using a 3-D convection–diffusion equation, the CR electrons and protons spectra in the two galactic regions are obtained and their spectra in the galactic disk are found to be flatter than the observed CR spectra. The diffuse gamma-ray spectrum produced by the interaction of these galactic CRs with the ISM and ISRFs is compared with the experimental data in both the galactic regions. Furthermore, the steepening of the observed local CR spectra from the galactic disk CR spectra are discussed by propagating local CRs having a source spectrum derived using local SNR age distribution (SNRs located within 1.5 kpc from the Sun), for a diffusion coefficient D0 ∼ 0.3 × 1027 cm2 s−1 in the local region which is much less than the typical value in the galaxy D0 ∼ (1–10) × 1028 cm2 s−1. The results obtained in this paper support the SNR origin of galactic CRs.

  • 291.
    Thoudam, Satyendra
    Bhabha Atomic Research Centre, India.
    On the contribution of nearby sources to the observed cosmic ray nuclei2008Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 388, nr 1, s. 335-346Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The presence of nearby discrete cosmic ray (CR) sources can lead to many interesting effects on the observed properties of CRs. In this paper, we study about the possible effects on the CR primary and secondary spectra and also the subsequent effects on the CR secondary-to-primary ratios. For the study, we assume that CRs undergo diffusive propagation in the Galaxy and we neglect the effect of convection, energy losses and reacceleration. In our model, we assume that there exists a uniform and continuous distribution of CR sources in the Galaxy generating a stationary CR background at the Earth. In addition, we also consider the existence of some nearby sources which inject CRs in a discrete space–time model. Assuming a constant CR source power throughout the Galaxy, our study has found that the presence of nearby supernova remnants (SNRs) produces noticeable variations in the primary fluxes mainly above ∼100 GeV n−1, if CRs are assumed to be released instantaneously after the supernova explosion. The variation reaches a value of ∼45 per cent at around 105 GeV n−1. Respect to earlier studies, the variation in the case of the secondaries is found to be almost negligible. We also discuss about the possible effects of the different particle release times from the SNRs. For the particle release time of ∼105 yr, predicted by the diffusive shock acceleration theories in SNRs, we have found that the presence of the nearby SNRs hardly produces any significant effects on the CRs at the Earth.

  • 292.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Propagation of cosmic rays in the galaxy and their measurements at very high energies with LORA2012Doktorsavhandling, monografi (Övrigt vetenskapligt)
  • 293.
    Thoudam, Satyendra
    Bhabha Atomic Research Centre, India.
    Revisiting the effect of nearby supernova remnants on local cosmic rays2007Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 380, nr 1, s. L1-L5Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In an earlier paper, the effect of the nearby known supernova remnants (SNRs) on the local cosmic rays (CRs) was studied, considering different possible forms of the particle injection time. The present work is a continuation of the previous work, but assumes a more realistic model of CR propagation in the Galaxy. The previous work assumed an unbounded three-dimensional diffusion region, whereas the present one considers a flat cylindrical disc bounded in both the radial and vertical directions. The study has found that the effect of the vertical halo boundary H on the local SNR contribution to the observed CR anisotropy is negligible as long as H≳ 2 kpc. Considering the values of the halo height H≳ 2 kpc obtained by different authors, the present work suggests that the study of the effect of local sources on the CR anisotropy can be carried out without having much information on H, and hence using the much simpler three-dimensional unbounded solution. Finally, the present work discusses the possibility of explaining the observed anisotropy below the knee by a single dominant source with properly chosen source parameters, and claims that the source may be an undetected old SNR with a characteristic age of ∼1.5 × 105 yr located at a distance of ∼0.57 kpc from the Sun.

  • 294.
    Thoudam, Satyendra
    et al.
    Radboud University Nijmegen, The Netherlands.
    Aar, G. V.
    Radboud University Nijmegen, The Netherlands.
    Akker, M. V. D.
    Radboud University Nijmegen, The Netherlands.
    Bähren, L.
    Radboud University Nijmegen, The Netherlands.
    Corstanje, A.
    Radboud University Nijmegen, The Netherlands.
    Falcke, H.
    Radboud University Nijmegen, The Netherlands ; ASTRON, The Netherlands.
    Hörandel, J. R.
    Radboud University Nijmegen, The Netherlands.
    Horneffer, A.
    Radboud University Nijmegen, The Netherlands.
    James, C.
    Radboud University Nijmegen, The Netherlands.
    Mevius, M.
    Kernfysisch Versneller Instituut, The Netherlands.
    Scholten, O.
    Kernfysisch Versneller Instituut, The Netherlands.
    Singh, K.
    Radboud University Nijmegen, The Netherlands ; Kernfysisch Versneller Instituut, The Netherlands ; Vrije Universiteit Brussel, Belgium.
    Ter Veen, S.
    Radboud University Nijmegen, The Netherlands.
    An air shower array for LOFAR: LORA2011Ingår i: Astrophysics and Space Sciences Transactions (ASTRA), ISSN 1810-6528, E-ISSN 1810-6536, Vol. 7, s. 195-199Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    LOFAR is a new form of radio telescope which can detect radio emission from air showers induced by very high-energy cosmic rays. It can also look for radio emission from particle cascades on the Moon induced by ultra high-energy cosmic rays or neutrinos. To complement the radio detection, we are setting up a small particle detector array LORA (LOfar Radboud Air shower array) within an area of ~300 m diameter in the LOFAR core. It will help in triggering and confirming the radio detection of air showers with the LOFAR antennas. In this paper, we present a short overview about LORA and discuss its current status.

  • 295.
    Thoudam, Satyendra
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Becherini, Yvonne
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Punch, Michael
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Simulation study for the proposed wide field-of-view gamma-ray detector array ALTO2018Ingår i: Proceedings of Science: 35th International Cosmic Ray Conference, ICRC 2017, 10 -20 July 2017, Bexico, South Korea, Sissa Medialab Srl , 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    ALTO is a wide field-of-view air shower detector array for very-high-energy (VHE) gamma-ray astronomy, proposed to be installed in the Southern Hemisphere at an altitude of ∼5.1 km above sea level. The array will use water Cherenkov detectors, as in the HAWC observatory, but combined with scintillator detectors, to detect air showers induced by VHE gamma rays in the atmosphere. It is being designed to attain a lower energy threshold, better energy and angular resolution, and improved sensitivity. The array will consist of ∼1250 small-sized (3.6 m diameter) detector units distributed over a circular area of ∼160 m in diameter. Each detector unit will consist of a water Cherenkov detector and a liquid scintillation detector underneath which will preferentially identify muons, facilitating the background (cosmic ray) rejection, thereby improving the sensitivity. The background rejection will be further enhanced by the close-packed arrangement and the small size of the detectors which will allow a fine sampling of air-shower footprints at the ground. In this contribution, we present the Monte-Carlo simulation of the experiment performed using CORSIKA and GEANT4 simulation packages. The expected performance of the array in terms of reconstruction accuracies of the shower core and arrival direction, as well as preliminary estimate of the trigger energy threshold after preliminary selection cuts for a point-like gamma-ray source are presented. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0).

  • 296.
    Thoudam, Satyendra
    et al.
    Radboud University Nijmegen, The Netherlands.
    Buitink, S.
    Radboud University Nijmegen, The Netherlands ; University of Groningen, The Netherlands.
    Corstanje, A.
    Radboud University Nijmegen, The Netherlands.
    Enriquez, J. E.
    Radboud University Nijmegen, The Netherlands.
    Falcke, H.
    Radboud University Nijmegen, The Netherlands ; ASTRON, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Frieswijk, W.
    ASTRON, The Netherlands.
    Hörandel, J. R.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Horneffer, A.
    Radboud University Nijmegen, The Netherlands.
    Krause, M.
    Radboud University Nijmegen, The Netherlands.
    Nelles, A.
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Schellart, P.
    Radboud University Nijmegen, The Netherlands.
    Scholten, O.
    University of Groningen, The Netherlands.
    ter Veen, S.
    Radboud University Nijmegen, The Netherlands.
    van den Akker, M.
    Radboud University Nijmegen, The Netherlands.
    LORA: A scintillator array for LOFAR to measure extensive air showers2014Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 767, s. 339-346Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The measurement of the radio emission from extensive air showers, induced by high-energy cosmic rays, is one of the key science projects of the LOFAR radio telescope. The LOfar Radboud air shower Array (LORA) has been installed in the core of LOFAR in the Netherlands. The main purpose of LORA is to measure the properties of air showers and to trigger the read-out of the LOFAR radio antennas to register extensive air showers. The experimental set-up of the array of scintillation detectors and its performance are described.

  • 297.
    Thoudam, Satyendra
    et al.
    Radboud University Nijmegen, The Netherlands.
    Buitink, S.
    Vrije Universiteit Brussel.
    Corstanje, A.
    Radboud University Nijmegen, The Netherlands.
    Enriquez, J. E.
    Radboud University Nijmegen, The Netherlands.
    Falcke, H.
    Radboud University Nijmegen, The Netherlands ; NIKHEF, Science Park Amsterdam, The Netherlands ; Netherlands Institute of Radio Astronomy (ASTRON), The Netherlands .
    Hörandel, J. R.
    Radboud University Nijmegen, The Netherlands ; NIKHEF, Science Park Amsterdam, The Netherlands.
    Nelles, A.
    Radboud University Nijmegen, The Netherlands ; University of California Irvine, USA.
    Rachen, J. P.
    Radboud University Nijmegen, The Netherlands.
    Rossetto, L.
    Radboud University Nijmegen, The Netherlands.
    Schellart, P.
    Radboud University Nijmegen, The Netherlands.
    Scholten, O.
    University Groningen, The Netherlands.
    ter Veen, S.
    Radboud University Nijmegen, The Netherlands.
    Trinh, T. N. G.
    University Groningen, The Netherlands.
    van Kessel, L.
    Radboud University Nijmegen, The Netherlands.
    Measurement of the cosmic-ray energy spectrum above 1016 eV with the LOFAR Radboud Air Shower Array2016Ingår i: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 73, s. 34-43Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The energy reconstruction of extensive air showers measured with the LOFAR Radboud Air Shower Array (LORA) is presented in detail. LORA is a particle detector array located in the center of the LOFAR radio telescope in the Netherlands. The aim of this work is to provide an accurate and independent energy measurement for the air showers measured through their radio signal with the LOFAR antennas. The energy reconstruction is performed using a parameterized relation between the measured shower size and the cosmic-ray energy obtained from air shower simulations. In order to illustrate the capabilities of LORA, the all-particle cosmic-ray energy spectrum has been reconstructed, assuming that cosmic rays are composed only of protons or iron nuclei in the energy range between ∼2 × 1016 and 2 × 1018 eV. The results are compatible with literature values and a changing mass composition in the transition region from a Galactic to an extragalactic origin of cosmic rays.

  • 298.
    Thoudam, Satyendra
    et al.
    Radboud University Nijmegen, The Netherlands.
    Hörandel, Jörg
    Science Park Amsterdam, The Netherlands.
    Anomaly in the cosmic-ray energy spectrum at GeV-TeV energies2015Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 632, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent measurements of cosmic rays by various experiments have found that the energy spectrum of cosmic rays is harder in the TeV region than at GeV energies. The origin of the spectral hardening is not clearly understood. In this paper, we discuss the possibility that the spectral hardening might be due to the effect of re-acceleration of cosmic rays by weak shocks associated with old supernova remnants in the Galaxy.

  • 299.
    Thoudam, Satyendra
    et al.
    Radboud University Nijmegen, The Netherlands.
    Hörandel, Jörg R.
    Radboud University Nijmegen, The Netherlands.
    A possible correlation between the high-energy electron spectrum and the cosmic ray secondary-to-primary ratios2011Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, s. 1432-1438Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent observations of high-energy cosmic ray electrons by the Fermi-Large Area Telescope (LAT) and the High Energy Stereoscopic System (HESS) experiments between 20 GeV and 5 TeV have found that the energy spectrum closely follows a broken power law with a break at around 1 TeV. On the other hand, measurements of cosmic ray secondary-to-primary ratios like the boron-to-carbon ratio seem to indicate a possible change in the slope at energies around 100 GeV n−1. In this paper, we discuss one possible explanation for the observed break in the electron spectrum and its possible correlation with the flattening in the secondary-to-primary ratios at higher energies. In our model, we assume that cosmic rays after acceleration by supernova remnant shock waves, escape downstream of the shock and remain confined within the remnant until the shock slows down. During this time, the high-energy electrons suffer from radiative energy losses and the cosmic ray nuclei undergo nuclear fragmentations due to their interactions with the matter. Once the cosmic rays are released from the supernova remnants, they follow diffusive propagation in the Galaxy where they further suffer from radiative or fragmentation losses.

  • 300.
    Thoudam, Satyendra
    et al.
    Radboud University Nijmegen, The Netherlands.
    Hörandel, Jörg R
    Radboud University Nijmegen, The Netherlands ; Science Park Amsterdam, The Netherlands.
    Cosmic-ray spectral anomaly at GeV-TeV energies as due to re-acceleration by weak shocks in the Galaxy2013Ingår i: Proceedings of the 33rd International Cosmic Rays Conference: The Astroparticle Physics Conference, 2013, s. 1-4Konferensbidrag (Refereegranskat)
    Abstract [en]

    Recent cosmic-ray measurements by the ATIC, CREAM and PAMELA experiments have found an apparent hardening of the energy spectrum at TeV energies. Although the origin of the hardening is not clearly understood, possible explanations include hardening in the cosmic-ray source spectrum, changes in the cosmic-ray propagation properties in the Galaxy and the effect of nearby sources. In this contribution, we propose that the spectral anomaly might be an effect of re-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration by strong supernova remnant shock waves, cosmic rays undergo diffusive propagation through the Galaxy. During the propagation, cosmic rays may again encounter expanding supernova remnant shock waves, and get re-accelerated. As the probability of encountering old supernova remnants is expected to be larger than the young ones due to their bigger size, re-acceleration is expected to be produced mainly by weaker shocks. Since weaker shocks generate a softer particle spectrum, the resulting re-accelerated component will have a spectrum steeper than the initial cosmic-ray source spectrum produced by strong shocks. For a reasonable set of model parameters, it is shown that such re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, explaining the observed GeV-TeV spectral anomaly.

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