Search for an extended VHE γ-ray emission from Mrk 421 and Mrk 501 with the MAGIC Telescope

¹ IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
² INAF National Institute for Astrophysics, 00136 Rome, Italy
³ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁴ Technische Universität Dortmund, 44221 Dortmund, Germany
⁵ Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
⁶ Universidad Complutense, 28040 Madrid, Spain
⁷ Università di Padova and INFN, 35131 Padova, Italy
⁸ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁹ University of Łódź, 90236 Lodz, Poland
¹⁰ Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² ETH Zurich, 8093 Switzerland
¹³ Max-Planck-Institut für Physik, 80805 München, Germany
¹⁴ Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
¹⁵ Universität Würzburg, 97074 Würzburg, Germany
¹⁶ Depto. de Astrofisica, Universidad, 38206 La Laguna, Tenerife, Spain
¹⁷ Università di Udine, and INFN Trieste, 33100 Udine, Italy
¹⁸ Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
¹⁹ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
²⁰ Croatian MAGIC Consortium, Institute R. Boskovic, University of Rijeka, University of Split, 10000 Zagreb, Croatia
²¹ University of California, Davis, 95616-8677, USA
²² Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²³ INAF/Osservatorio Astronomico and INFN, 34143 Trieste, Italy
²⁴ ICREA, 08010 Barcelona, Spain
²⁵ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁶ SDC Data Center for Astrophysics, Geneva Observatory, Chemin d’Écogia 16, 1290 Versoix, Switzerland
²⁷ APC, 10 rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13, France

^† Corresponding author: R. Mirzoyan, e-mail: razmik@mppmu.mpg.de
^‡ Corresponding author: J. Sitarek, e-mail: jsitarek@mppmu.mpg.de

Received: 7 April 2010
Accepted: 12 July 2010

Abstract

Context. Part of the very high energy γ-ray radiation coming from extragalactic sources is absorbed through the pair production process on the extragalactic background light photons. Extragalactic magnetic fields alter the trajectories of these cascade pairs and, in turn, convert cosmic background photons to γ-ray energies by inverse Compton scattering. These secondary photons can form an extended halo around bright VHE sources.

Aims. We searched for an extended emission around the bright blazars Mrk 421 and Mrk 501 using the MAGIC telescope data.

Methods. If extended emission is present, the angular distribution of reconstructed γ-ray arrival directions around the source is broader than for a point-like source. In the analysis of a few tens of hours of observational data taken from Mrk 421 and Mrk 501 we used a newly developed method that provides better angular resolution. This method is based on the usage of multidimensional decision trees. Comparing the measured shapes of angular distributions with those expected from a point-like source one can detect or constrain possible extended emission around the source. We also studied the influence of different types of systematic errors on the shape of the distribution of reconstructed γ-ray arrival directions for a point source.

Results. We present upper limits for an extended emission calculated for both sources for various source extensions and emission profiles. We obtain upper limits on the extended emission around the Mrk 421 (Mrk 501) on the level of <5% (<4%) of the Crab Nebula flux above the energy threshold of 300 GeV. Using these results we discuss possible constraints on the extragalactic magnetic fields strength around a few times 10^-15 G.