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Abdalla, H., Adam, R., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., . . . Meyer, M. (2019). Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE gamma-ray observations with HESS. Astronomy and Astrophysics, 627, 1-19, Article ID A159.
Open this publication in new window or tab >>Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE gamma-ray observations with HESS
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, p. 1-19, article id A159Article in journal (Refereed) Published
Abstract [en]

The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100MeV < E < 100 GeV) gamma-ray band, which is continuously monitored with Fermi-LAT. During two periods of high activity in April 2014 and June 2015 target-of-opportunity observations were undertaken with the High Energy Stereoscopic System (H.E.S.S.) in the very-high-energy (VHE, E > 100 GeV) gamma-ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with 8 : 7 sigma significance above an energy threshold of 66 GeV. No VHE variability was detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of 4.2 +/- 0.3. The H.E.S.S. data along with a detailed and contemporaneous multiwavelength data set provide constraints on the physical parameters of the emission region. The minimum distance of the emission region from the central black hole was estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at r greater than or similar to 1 : 7 X 1017 cm from the black hole, that is beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models were used to describe the evolution of the 2015 flare. Neither model can fully reproduce the observations, despite testing various parameter sets. Furthermore, the H.E.S.S. data were used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
radiation mechanisms: non-thermal, quasars: individual: 3C 279, galaxies: active, relativistic processes
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-87058 (URN)10.1051/0004-6361/201935704 (DOI)000475554500004 ()2-s2.0-85069435438 (Scopus ID)
Available from: 2019-08-01 Created: 2019-08-01 Last updated: 2019-08-29Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., Arcaro, C., . . . Zywucka, N. (2019). HESS and Suzaku observations of the Vela X pulsar wind nebula. Astronomy and Astrophysics, 627, 1-16, Article ID A100.
Open this publication in new window or tab >>HESS and Suzaku observations of the Vela X pulsar wind nebula
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, p. 1-16, article id A100Article in journal (Refereed) Published
Abstract [en]

Context. Pulsar wind nebulae (PWNe) represent the most prominent population of Galactic very-high-energy gamma-ray sources and are thought to be an efficient source of leptonic cosmic rays. Vela X is a nearby middle-aged PWN, which shows bright X-ray and TeV gamma-ray emission towards an elongated structure called the cocoon. Aims. Since TeV emission is likely inverse-Compton emission of electrons, predominantly from interactions with the cosmic microwave background, while X-ray emission is synchrotron radiation of the same electrons, we aim to derive the properties of the relativistic particles and of magnetic fields with minimal modelling. Methods. We used data from the Suzaku XIS to derive the spectra from three compact regions in Vela X covering distances from 0.3 to 4 pc from the pulsar along the cocoon. We obtained gamma-ray spectra of the same regions from H.E.S.S. observations and fitted a radiative model to the multi-wavelength spectra. Results. The TeV electron spectra and magnetic field strengths are consistent within the uncertainties for the three regions, with energy densities of the order 10(-12) erg cm(-3). The data indicate the presence of a cutoff in the electron spectrum at energies of similar to 100 TeV and a magnetic field strength of similar to 6 mu G. Constraints on the presence of turbulent magnetic fields are weak. Conclusions. The pressure of TeV electrons and magnetic fields in the cocoon is dynamically negligible, requiring the presence of another dominant pressure component to balance the pulsar wind at the termination shock. Sub-TeV electrons cannot completely account for the missing pressure, which may be provided either by relativistic ions or from mixing of the ejecta with the pulsar wind. The electron spectra are consistent with expectations from transport scenarios dominated either by advection via the reverse shock or by diffusion, but for the latter the role of radiative losses near the termination shock needs to be further investigated in the light of the measured cutoff energies. Constraints on turbulent magnetic fields and the shape of the electron cutoff can be improved by spectral measurements in the energy range greater than or similar to 10 keV.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
stars: winds, outflows, gamma rays: stars, radiation mechanisms: non-thermal, acceleration of particles, pulsars: individual: PSR B0833-45
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-86983 (URN)10.1051/0004-6361/201935458 (DOI)000474334300001 ()2-s2.0-85069766171 (Scopus ID)
Available from: 2019-07-25 Created: 2019-07-25 Last updated: 2019-08-29Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., Arcaro, C., . . . Zywucka, N. (2019). HESS observations of the flaring gravitationally lensed galaxy PKS 1830-211. Monthly notices of the Royal Astronomical Society, 486(3), 3886-3891
Open this publication in new window or tab >>HESS observations of the flaring gravitationally lensed galaxy PKS 1830-211
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 486, no 3, p. 3886-3891Article in journal (Refereed) Published
Abstract [en]

PKS 1830-211 is a known macrolensed quasar located at a redshift of z = 2.5. Its highenergy gamma-ray emission has been detected with the Fermi-Large Area Telescope (LAT) instrument and evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830-211 were taken with the High Energy Stereoscopic System (H.E.S.S.) array of Imaging Atmospheric Cherenkov Telescopes in 2014 August, following a flare alert by the Fermi-LAT Collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20-27 d from the alert flare, as expected from observations at other wavelengths. More than 12 h of good-quality data were taken with an analysis threshold of similar to 67 GeV. The significance of a potential signal is computed as a function of the date and the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830-211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.

Place, publisher, year, edition, pages
Oxford University Press, 2019
Keywords
gravitational lensing: strong, diffuse radiation, gamma-rays: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-86986 (URN)10.1093/mnras/stz1031 (DOI)000474903500069 ()
Available from: 2019-07-25 Created: 2019-07-25 Last updated: 2019-07-25Bibliographically approved
Acharyya, A., Agudo, I., Anguner, E. O., Alfaro, R., Alfaro, J., Alispach, C., . . . Punch, M. (2019). Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout. Astroparticle physics, 111, 35-53
Open this publication in new window or tab >>Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout
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2019 (English)In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 111, p. 35-53Article in journal (Refereed) Published
Abstract [en]

The Cherenkov Telescope Array (CTA) is the major next-generation observatory for ground-based veryhigh-energy gamma-ray astronomy. It will improve the sensitivity of current ground-based instruments by a factor of five to twenty, depending on the energy, greatly improving both their angular and energy resolutions over four decades in energy (from 20 GeV to 300 TeV). This achievement will be possible by using tens of imaging Cherenkov telescopes of three successive sizes. They will be arranged into two arrays, one per hemisphere, located on the La Palma island (Spain) and in Paranal (Chile). We present here the optimised and final telescope arrays for both CTA sites, as well as their foreseen performance, resulting from the analysis of three different large-scale Monte Carlo productions.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Monte Carlo simulations, Cherenkov telescopes, IACT technique, Gamma rays, Cosmic rays
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-85847 (URN)10.1016/j.astropartphys.2019.04.001 (DOI)000470047300004 ()
Available from: 2019-06-25 Created: 2019-06-25 Last updated: 2019-08-27Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., Arcaro, C., . . . Zywucka, N. (2019). Particle transport within the pulsar wind nebula HESS J1825-137. Astronomy and Astrophysics, 621, Article ID A116.
Open this publication in new window or tab >>Particle transport within the pulsar wind nebula HESS J1825-137
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A116Article in journal (Refereed) Published
Abstract [en]

Context. We present a detailed view of the pulsar wind nebula (PWN) HESS J1825-137. We aim to constrain the mechanisms dominating the particle transport within the nebula, accounting for its anomalously large size and spectral characteristics. Aims. The nebula was studied using a deep exposure from over 12 years of H.E.S.S. I operation, together with data from H.E.S.S. II that improve the low-energy sensitivity. Enhanced energy-dependent morphological and spatially resolved spectral analyses probe the very high energy (VHE, E > 0.1 TeV) gamma-ray properties of the nebula. Methods. The nebula emission is revealed to extend out to 1.5 degrees from the pulsar, similar to 1.5 times farther than previously seen, making HESS J1825-137, with an intrinsic diameter of similar to 100 pc, potentially the largest gamma-ray PWN currently known. Characterising the strongly energy-dependent morphology of the nebula enables us to constrain the particle transport mechanisms. A dependence of the nebula extent with energy of R proportional to E alpha with alpha = -0.29 +/- 0.04(stat) +/- 0.05(sys) disfavours a pure diffusion scenario for particle transport within the nebula. The total gamma-ray flux of the nebula above 1 TeV is found to be (1.12 +/- 0.03(stat) +/- 0.25(sys)) +/- 10(-11) cm(-2) s(-1), corresponding to similar to 64% of the flux of the Crab nebula. Results. HESS J1825-137 is a PWN with clearly energy-dependent morphology at VHE gamma-ray energies. This source is used as a laboratory to investigate particle transport within intermediate-age PWNe. Based on deep observations of this highly spatially extended PWN, we produce a spectral map of the region that provides insights into the spectral variation within the nebula.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
gamma rays: general, acceleration of particles, convection, diffusion, pulsars: general
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-80151 (URN)10.1051/0004-6361/201834335 (DOI)000455572500001 ()2-s2.0-85060367011 (Scopus ID)
Available from: 2019-02-05 Created: 2019-02-05 Last updated: 2019-08-29Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguner, E. O., Arakawa, M., Arcaro, C., . . . Zywucka, N. (2019). The 2014TeV gamma-Ray Flare of Mrk 501 Seen with HESS: Temporal and Spectral Constraints on Lorentz Invariance Violation. Astrophysical Journal, 870(2), Article ID 93.
Open this publication in new window or tab >>The 2014TeV gamma-Ray Flare of Mrk 501 Seen with HESS: Temporal and Spectral Constraints on Lorentz Invariance Violation
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2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 870, no 2, article id 93Article in journal (Refereed) Published
Abstract [en]

The blazar Mrk 501 (z = 0.034) was observed at very-high-energy (VHE, E greater than or similar to 100 GeV) gamma-ray wavelengths during a bright flare on the night of 2014 June 23-24 (MJD 56832) with the H.E.S.S. phase-II array of Cherenkov telescopes. Data taken that night by H.E.S.S. at large zenith angle reveal an exceptional number of gamma-ray photons at multi-TeV energies, with rapid flux variability and an energy coverage extending significantly up to 20 TeV. This data set is used to constrain Lorentz invariance violation (LIV) using two independent channels: a temporal approach considers the possibility of an energy dependence in the arrival time of gamma-rays, whereas a spectral approach considers the possibility of modifications to the interaction of VHE gamma-rays with extragalactic background light (EBL) photons. The non-detection of energy-dependent time delays and the non-observation of deviations between the measured spectrum and that of a supposed power-law intrinsic spectrum with standard EBL attenuation are used independently to derive strong constraints on the energy scale of LIV (E-QG) in the subluminal scenario for linear and quadratic perturbations in the dispersion relation of photons. For the case of linear perturbations, the 95% confidence level limits obtained are E-QG,E-1 > 3.6 x 10(17) GeV using the temporal approach and E-QG,E-1 > 2.6 x 10(19) GeV using the spectral approach. For the case of quadratic perturbations, the limits obtained are E-QG,E-2 > 8.5 x 10(10) GeV using the temporal approach and E-QG,E-2 > 7.8 x 10(11) GeV using the spectral approach.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
astroparticle physics, BL Lacertae objects: individual (Mrk 501), gamma rays: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-80152 (URN)10.3847/1538-4357/aaf1c4 (DOI)000455820100039 ()2-s2.0-85060189217 (Scopus ID)
Available from: 2019-02-05 Created: 2019-02-05 Last updated: 2019-08-29Bibliographically approved
Abdalla, H., Aharonian, F., Benkhali, F. A., Anguener, E. O., Arakawa, M., Arcaro, C., . . . Maxted, N. ,. (2019). Upper limits on very-high-energy gamma-ray emission from core-collapse supernovae observed with HESS. Astronomy and Astrophysics, 626, 1-11, Article ID A57.
Open this publication in new window or tab >>Upper limits on very-high-energy gamma-ray emission from core-collapse supernovae observed with HESS