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  • 101.
    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).

  • 102. 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.

  • 103. 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.

  • 104.
    Thoudam, Satyendra
    et al.
    Radboud University Nijmegen, The Netherlands.
    Hörandel, Jörg R.
    Radboud University Nijmegen, The Netherlands.
    GeV-TeV cosmic-ray spectral anomaly as due to reacceleration by weak shocks in the Galaxy2014Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 567, s. 1-10, artikel-id A33Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent cosmic-ray measurements have found an anomaly in the cosmic-ray energy spectrum at GeV-TeV energies. Although the origin of the anomaly is not clearly understood, suggested explanations include the effect of cosmic-ray source spectrum, propagation effects, and the effect of nearby sources. In this paper, we propose that the spectral anomaly might be an effect of reacceleration 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 younger remnants because of their bigger sizes, reacceleration 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 the re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, thereby explaining the observed GeV-TeV spectral anomaly.

  • 105.
    Thoudam, Satyendra
    et al.
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE). Radboud Univ Nijmegen, Netherlands.
    Rachen, J. P.
    Radboud Univ Nijmegen, Netherlands.
    van Vliet, A.
    Radboud Univ Nijmegen, Netherlands.
    Achterberg, A.
    Radboud Univ Nijmegen, Netherlands.
    Buitink, S.
    Vrije Univ Brussel, Belgium.
    Falcke, H.
    Radboud Univ Nijmegen, Netherlands ; NIKHEF, Netherlands ; ASTRON, Netherlands.
    Horandel, J. R.
    Radboud Univ Nijmegen, Netherlands ; NIKHEF, Netherlands.
    Cosmic-ray energy spectrum and composition up to the ankle: the case for a second Galactic component2016Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 595, artikel-id A33Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Motivated by the recent high-precision measurements of cosmic rays by several new-generation experiments, we have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to about 10(18) eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of different elements, optimised to explain the observed elemental spectra below similar to 10(14) eV and the "knee" in the all-particle spectrum, cannot explain the observed all-particle spectrum above similar to 2 x 10(16) eV. We discuss two approaches for a second component of Galactic cosmic rays re-acceleration at a Galactic wind termination shock, and supernova explosions of Wolf-Rayet stars, and show that the latter scenario can explain almost all observed features in the all-particle spectrum and the composition up to similar to 10(18) eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. In this two-component Galactic model, the knee at similar to 3 x 10(15) eV and the "second knee" at similar to 10(17) eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. We also discuss several variations of the extra-galactic component, from a minimal contribution to scenarios with a significant component below the "ankle" (at similar to 4 x 10(18) eV), and find that extragalactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. We also provide arguments that an extra-galactic contribution is unlikely to dominate at or below the second knee. Our main result is that the second Galactic component predicts a composition of Galactic cosmic rays at and above the second knee that largely consists of helium or a mixture of helium and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast to most common assumptions. This prediction is in agreement with new measurements from LOFAR and the Pierre Auger Observatory which indicate a strong light component and a rather low iron fraction between similar to 10(17) and 10(18) eV.

  • 106. van Haarlem, M. P.
    et al.
    Wise, M. W.
    Gunst, A. W.
    Heald, G.
    McKean, J. P.
    Hessels, J. W. T.
    de Bruyn, A. G.
    Nijboer, R.
    Swinbank, J.
    Fallows, R.
    Brentjens, M.
    Nelles, A.
    Beck, R.
    Falcke, H.
    Fender, R.
    Hörandel, J.
    Koopmans, L. V. E.
    Mann, G.
    Miley, G.
    Röttgering, H.
    Stappers, B. W.
    Wijers, R. A. M. J.
    Zaroubi, S.
    van den Akker, M.
    Alexov, A.
    Anderson, J.
    Anderson, K.
    van Ardenne, A.
    Arts, M.
    Asgekar, A.
    Avruch, I. M.
    Batejat, F.
    Bähren, L.
    Bell, M. E.
    Bell, M. R.
    van Bemmel, I.
    Bennema, P.
    Bentum, M. J.
    Bernardi, G.
    Best, P.
    Bîrzan, L.
    Bonafede, A.
    Boonstra, A. -J
    Braun, R.
    Bregman, J.
    Breitling, F.
    van de Brink, R. H.
    Broderick, J.
    Broekema, P. C.
    Brouw, W. N.
    Brüggen, M.
    Butcher, H. R.
    van Cappellen, W.
    Ciardi, B.
    Coenen, T.
    Conway, J.
    Coolen, A.
    Corstanje, A.
    Damstra, S.
    Davies, O.
    Deller, A. T.
    Dettmar, R. -J
    van Diepen, G.
    Dijkstra, K.
    Donker, P.
    Doorduin, A.
    Dromer, J.
    Drost, M.
    van Duin, A.
    Eislöffel, J.
    van Enst, J.
    Ferrari, C.
    Frieswijk, W.
    Gankema, H.
    Garrett, M. A.
    de Gasperin, F.
    Gerbers, M.
    de Geus, E.
    Grießmeier, J. -M
    Grit, T.
    Gruppen, P.
    Hamaker, J. P.
    Hassall, T.
    Hoeft, M.
    Holties, H. A.
    Horneffer, A.
    van der Horst, A.
    van Houwelingen, A.
    Huijgen, A.
    Iacobelli, M.
    Intema, H.
    Jackson, N.
    Jelic, V.
    de Jong, A.
    Juette, E.
    Kant, D.
    Karastergiou, A.
    Koers, A.
    Kollen, H.
    Kondratiev, V. I.
    Kooistra, E.
    Koopman, Y.
    Koster, A.
    Kuniyoshi, M.
    Kramer, M.
    Kuper, G.
    Lambropoulos, P.
    Law, C.
    van Leeuwen, J.
    Lemaitre, J.
    Loose, M.
    Maat, P.
    Macario, G.
    Markoff, S.
    Masters, J.
    McFadden, R. A.
    McKay-Bukowski, D.
    Meijering, H.
    Meulman, H.
    Mevius, M.
    Middelberg, E.
    Millenaar, R.
    Miller-Jones, J. C. A.
    Mohan, R. N.
    Mol, J. D.
    Morawietz, J.
    Morganti, R.
    Mulcahy, D. D.
    Mulder, E.
    Munk, H.
    Nieuwenhuis, L.
    van Nieuwpoort, R.
    Noordam, J. E.
    Norden, M.
    Noutsos, A.
    Offringa, A. R.
    Olofsson, H.
    Omar, A.
    Orrú, E.
    Overeem, R.
    Paas, H.
    Pandey-Pommier, M.
    Pandey, V. N.
    Pizzo, R.
    Polatidis, A.
    Rafferty, D.
    Rawlings, S.
    Reich, W.
    de Reijer, J. -P
    Reitsma, J.
    Renting, G. A.
    Riemers, P.
    Rol, E.
    Romein, J. W.
    Roosjen, J.
    Ruiter, M.
    Scaife, A.
    van der Schaaf, K.
    Scheers, B.
    Schellart, P.
    Schoenmakers, A.
    Schoonderbeek, G.
    Serylak, M.
    Shulevski, A.
    Sluman, J.
    Smirnov, O.
    Sobey, C.
    Spreeuw, H.
    Steinmetz, M.
    Sterks, C. G. M.
    Stiepel, H. -J
    Stuurwold, K.
    Tagger, M.
    Tang, Y.
    Tasse, C.
    Thomas, I.
    Thoudam, Satyendra
    Radboud University Nijmegen, The Netherlands.
    Toribio, M. C.
    van der Tol, B.
    Usov, O.
    van Veelen, M.
    van der Veen, A. -J
    ter Veen, S.
    Verbiest, J. P. W.
    Vermeulen, R.
    Vermaas, N.
    Vocks, C.
    Vogt, C.
    de Vos, M.
    van der Wal, E.
    van Weeren, R.
    Weggemans, H.
    Weltevrede, P.
    White, S.
    Wijnholds, S. J.
    Wilhelmsson, T.
    Wucknitz, O.
    Yatawatta, S.
    Zarka, P.
    Zensus, A.
    van Zwieten, J.
    LOFAR: The LOw-Frequency ARray2013Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, s. 1-53, artikel-id A2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10–240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR’s new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

  • 107.
    Zucca, P.
    et al.
    ASTRON, Netherlands.
    Morosan, D. E.
    Trinity Coll Dublin, Ireland.
    Rouillard, A. P.
    Inst Rech Astrophys & Planetol, France.
    Fallows, R.
    ASTRON, Netherlands.
    Gallagher, P. T.
    Trinity Coll Dublin, Ireland.
    Magdalenic, J.
    Royal Observ Belgium, Belgium.
    Klein, K-L
    Observ Paris, France.
    Mann, G.
    Leibniz Inst Astrophys Potsdam AIP, Germany.
    Vocks, C.
    Leibniz Inst Astrophys Potsdam AIP, Germany.
    Carley, E. P.
    Trinity Coll Dublin, Ireland.
    Bisi, M. M.
    RAL Space, UK.
    Kontar, E. P.
    Univ Glasgow, UK.
    Rothkaehl, H.
    Polish Acad Sci, Poland.
    Dabrowski, B.
    Univ Warmia & Mazury, Poland.
    Krankowski, A.
    Univ Warmia & Mazury, Poland.
    Anderson, J.
    Helmholtz Zentrum Potsdam, Germany.
    Asgekar, A.
    ASTRON, Netherlands;Shell Technol Ctr, India.
    Bell, M. E.
    Univ Technol Sydney, Australia.
    Bentum, M. J.
    ASTRON, Netherlands;Eindhoven Univ Technol, Netherlands.
    Best, P.
    Univ Edinburgh, UK.
    Blaauw, R.
    ASTRON, Netherlands.
    Breitling, F.
    Leibniz Inst Astrophys Potsdam AIP, Germany.
    Broderick, J. W.
    ASTRON, Netherlands.
    Brouw, W. N.
    ASTRON, Netherlands;Kapteyn Astron Inst, Netherlands.
    Brueggen, M.
    Univ Hamburg, Germany.
    Butcher, H. R.
    Australian Natl Univ, Australia.
    Ciardi, B.
    Max Planck Inst Astrophys, Germany.
    de Geus, E.
    ASTRON, Netherlands;SmarterVision BV, Netherlands.
    Deller, A.
    ASTRON, Netherlands;Swinburne Univ Technol, Australia.
    Duscha, S.
    ASTRON, Netherlands.
    Eisloeffel, J.
    Thuringer Landessternwarte,Germany.
    Garrett, M. A.
    Univ Manchester, UK;Leiden Univ, Netherlands.
    Griessmeier, J. M.
    Univ Orleans, France;CNRS, France.
    Gunst, A. W.
    ASTRON, Netherlands.
    Heald, G.
    ASTRON, Netherlands;CSIRO Astron & Space Sci, Australia.
    Hoeft, M.
    Eindhoven Univ Technol, Netherlands.
    Horandel, J.
    Radboud Univ Nijmegen, Netherlands.
    Iacobelli, M.
    ASTRON, Netherlands.
    Juette, E.
    Ruhr Univ Bochum, Germany.
    Karastergiou, A.
    Univ Oxford, UK.
    van Leeuwen, J.
    Trinity Coll Dublin, Ireland;Univ Amsterdam, Netherlands.
    McKay-Bukowski, D.
    Univ Tromso, Norway;STFC Rutherford Appleton Lab, UK.
    Mulder, H.
    ASTRON, Netherlands.
    Munk, H.
    ASTRON, Netherlands;Radboud Univ Nijmegen, Netherlands.
    Nelles, A.
    Univ Calif Irvine, USA.
    Orru, E.
    ASTRON, Netherlands.
    Paas, H.
    Univ Groningen, Netherlands.
    Pandey, V. N.
    ASTRON, Netherlands;Observ Paris, France.
    Pekal, R.
    Poznan Supercomp & Networking Ctr PCSS, Poland.
    Pizzo, R.
    ASTRON, Netherlands.
    Polatidis, A. G.
    ASTRON, Netherlands.
    Reich, W.
    Max Planck Inst Radioastron, Germany.
    Rowlinson, A.
    ASTRON, Netherlands.
    Schwarz, D. J.
    Univ Bielefeld, Germany.
    Shulevski, A.
    Kapteyn Astron Inst, Netherlands.
    Sluman, J.
    ASTRON, Netherlands.
    Smirnov, O.
    Rhodes Univ, South Africa;SKA South Africa, South Africa.
    Sobey, C.
    Curtin Univ, Australia.
    Soida, M.
    Jagiellonian Univ, Poland.
    Thoudam, Satyendra
    Linnéuniversitetet, Fakulteten för teknik (FTK), Institutionen för fysik och elektroteknik (IFE).
    Toribio, M. C.
    ASTRON, Netherlands;Kapteyn Astron Inst, Netherlands.
    Vermeulen, R.
    ASTRON, Netherlands.
    van Weeren, R. J.
    Kapteyn Astron Inst, Netherlands.
    Wucknitz, O.
    Max Planck Inst Radioastron, Germany.
    Zarka, P.
    Observ Paris, France.
    Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 615, artikel-id A89Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with coronal mass ejections (CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20-90 MHz) is now possible with the Low Frequency Array (LOFAR), opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs. Aims. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon. Methods. The type II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of LOFAR, and the CME was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) and by the COR2A coronagraph of the SECCHI instruments on board the Solar Terrestrial Relation Observatory (STEREO). The 3D structure was inferred using triangulation of the coronographic observations. Coronal magnetic fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic model using the photospheric fields measured by the Heliospheric Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary. Results. The type II radio source of the coronal shock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with theta(Bn)similar to 70 degrees. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that radio wave propagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3-1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the coronal magnetic field around the type II source were found to lie in the range 1.4-1.6.

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