MAGIC observations and multifrequency properties of the flat spectrum radio quasar 3C 279 in 2011

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 Łódź, 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 Università di Padova and INFN, 35131 Padova, Italy
¹⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁶ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
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 Japanese MAGIC Consortium, Division of Physics and Astronomy, Kyoto University, 606-8501 Kyoto, Japan
²¹ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, 900147 Oulu, Finland
²² Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²³ Universitat de Barcelona (ICC, IEEC-UB), 08028 Barcelona, Spain
²⁴ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁵ Now at École polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
²⁶ Now at Department of Physics & Astronomy, UC Riverside, CA 92521, USA
²⁷ Now at Finnish Centre for Astronomy with ESO (FINCA), Turku, Finland
²⁸ Also at INAF-Trieste  
²⁹ Also at Instituto de Fisica Teorica, UAM/CSIC, 28049 Madrid, Spain
³⁰ Now at: Stockholm University, Oskar Klein Centre for Cosmoparticle Physics, 106 91 Stockholm, Sweden
³¹ Now at GRAPPA Institute, University of Amsterdam, 1098XH Amsterdam, The Netherlands
³² Department of Physics and Astronomy, University of Turku, Finland
³³ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540, Kylmälä, Finland
³⁴ Cahill Center for Astronomy & Astrophysics, Caltech, 1200 E. California Blvd, Pasadena, CA 91125, USA
³⁵ Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
³⁶ KIPAC, SLAC National Accelerator Laboratory, Stanford, CA 94025, USA
³⁷ Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
³⁸ Institute for Astrophysical Research, Boston University, USA

Received: 12 November 2013
Accepted: 24 February 2014

Abstract

Aims. We study the multifrequency emission and spectral properties of the quasar3C 279 aimed at identifying the radiation processes taking place in the source.

Methods. We observed 3C 279 in very-high-energy (VHE, E> 100 GeV) γ-rays, with the MAGIC telescopes during 2011, for the first time in stereoscopic mode. We combined these measurements with observations at other energy bands: in high-energy (HE, E> 100 MeV) γ-rays from Fermi-LAT; in X-rays from RXTE; in the optical from the KVA telescope; and in the radio at 43 GHz, 37 GHz, and 15 GHz from the VLBA, Metsähovi, and OVRO radio telescopes – along with optical polarisation measurements from the KVA and Liverpool telescopes. We examined the corresponding light curves and broadband spectral energy distribution and we compared the multifrequency properties of 3C 279 at the epoch of the MAGIC observations with those inferred from historical observations.

Results. During the MAGIC observations (2011 February 8 to April 11) 3C 279 was in a low state in optical, X-ray, and γ-rays. The MAGIC observations did not yield a significant detection. The derived upper limits are in agreement with the extrapolation of the HE γ-ray spectrum, corrected for EBL absorption, from Fermi-LAT. The second part of the MAGIC observations in 2011 was triggered by a high-activity state in the optical and γ-ray bands. During the optical outburst the optical electric vector position angle (EVPA) showed a rotation of ~180°. Unlike previous cases, there was no simultaneous rotation of the 43 GHz radio polarisation angle. No VHE γ-rays were detected by MAGIC, and the derived upper limits suggest the presence of a spectral break or curvature between the Fermi-LAT and MAGIC bands. The combined upper limits are the strongest derived to date for the source at VHE and below the level of the previously detected flux by a factor of ~2. Radiation models that include synchrotron and inverse Compton emissions match the optical to γ-ray data, assuming an emission component inside the broad line region with size R = 1.1 × 10¹⁶ cm and magnetic field B = 1.45 G responsible for the high-energy emission, and another one outside the broad line region and the infrared torus (R = 1.5 × 10¹⁷ cm and B = 0.8 G) causing the optical and low-energy emission. We also study the optical polarisation in detail and interpret it with a bent trajectory model.