Volume 564, April 2014
|Number of page(s)||13|
|Published online||27 March 2014|
Contemporaneous observations of the radio galaxy NGC 1275 from radio to very high energy γ-rays
IFAE, Edifici Cn., Campus UAB, 08193
2 Università di Udine, and INFN Trieste, 33100 Udine, Italy
3 INAF National Institute for Astrophysics, 00136 Rome, Italy
4 Università di Siena, and INFN Pisa, 53100 Siena, Italy
5 Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
6 Max-Planck-Institut für Physik, 80805 München, Germany
7 Universidad Complutense, 28040 Madrid, Spain
8 Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
9 University of Lodz, 90236 Lodz, Poland
10 Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
11 ETH Zurich, 8093 Zurich, Switzerland
12 Universität Würzburg, 97074 Würzburg, Germany
13 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
14 Technische Universität Dortmund, 44221 Dortmund, Germany
15 Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
16 Università di Padova and INFN, 35131 Padova, Italy
17 Università dell’Insubria, Como, 22100 Como, Italy
18 Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
19 Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
20 Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, 900147 Oulu Finland
21 Japanese MAGIC Consortium, Division of Physics and Astronomy, Kyoto University, Japan
22 Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
23 Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
24 Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
25 Now at École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
26 Now at Department of Physics & Astronomy, UC Riverside CA 92521, USA
27 Now at Finnish Centre for Astronomy with ESO (FINCA), 21500 Turku, Finland
28 also at Instituto de Fisica Teorica, UAM/CSIC, 28049 Madrid, Spain
29 Now at Stockholms universitet, Oskar Klein Centre for Cosmoparticle Physics, Stockholm University, 10691 Stockholm Sweden
30 Now at GRAPPA Institute, University of Amsterdam, 1098XH Amsterdam, The Netherlands
31 INAF - Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
32 Waseda University, 169-8050, Tokyo, Japan
Accepted: 23 January 2014
Aims. The radio galaxy NGC 1275, recently identified as a very high energy (VHE, >100 GeV) γ-ray emitter by MAGIC, is one of the few non-blazar active galactic nuclei detected in the VHE regime. The purpose of this work is to better understand the origin of the γ-ray emission and locate it within the galaxy.
Methods. We studied contemporaneous multifrequency observations of NGC 1275 and modeled the overall spectral energy distribution. We analyzed unpublished MAGIC observations carried out between October 2009 and February 2010, and the previously published observations taken between August 2010 and February 2011. We studied the multiband variability and correlations by analyzing data of Fermi-LAT in the 100 MeV–100 GeV energy band, as well as Chandra (X-ray), KVA (optical), and MOJAVE (radio) data taken during the same period.
Results. Using customized Monte Carlo simulations corresponding to early MAGIC stereoscopic data, we detect NGC 1275 also in the earlier MAGIC campaign. The flux level and energy spectra are similar to the results of the second campaign. The monthly light curve above 100 GeV shows a hint of variability at the 3.6σ level. In the Fermi-LAT band, both flux and spectral shape variabilities are reported. The optical light curve is also variable and shows a clear correlation with the γ-ray flux above 100 MeV. In radio, three compact components are resolved in the innermost part of the jet. One of these components shows a similar trend as the Fermi-LAT and KVA light curves. The γ-ray spectra measured simultaneously with MAGIC and Fermi-LAT from 100 MeV to 650 GeV can be well fitted either by a log-parabola or by a power-law with a subexponential cutoff for the two observation campaigns. A single-zone synchrotron-self-Compton model, with an electron spectrum following a power-law with an exponential cutoff, can explain the broadband spectral energy distribution and the multifrequency behavior of the source. However, this model suggests an untypical low bulk-Lorentz factor or a velocity alignment closer to the line of sight than the parsec-scale radio jet.
Key words: galaxies: active / gamma rays: galaxies / galaxies: jets / galaxies: individual: NGC 1275
© ESO, 2014
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