Contemporaneous observations of the radio galaxy NGC 1275 from radio to very high energy γ-rays

¹ IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
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

Received: 28 October 2013
Accepted: 23 January 2014

Abstract

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.