Detection of very-high energy γ-ray emission from NGC 1275 by the MAGIC telescopes

¹ IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
² Universidad Complutense, 28040 Madrid, Spain
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
⁶ Max-Planck-Institut für Physik, 80805 München, Germany
e-mail: colin@mppmu.mpg.de
⁷ Università di Padova and INFN, 35131 Padova, Italy
e-mail: saverio.lombardi@pd.infn.it
⁸ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁹ Depto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
¹⁰ University of Łódź, 90236 Lodz, Poland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² ETH Zurich, 8093 Zurich, Switzerland
e-mail: dorothee.hildebrand@phys.ethz.ch
¹³ Universität Würzburg, 97074 Würzburg, Germany
¹⁴ Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
¹⁵ Università di Udine, and INFN Trieste, 33100 Udine, Italy
¹⁶ Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
¹⁷ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
¹⁸ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
e-mail: fabio@iaa.es
¹⁹ Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁰ Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
²¹ Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²² INAF/Osservatorio Astronomico and INFN, 34143 Trieste, Italy
²³ Università dell’Insubria, Como, 22100 Como, Italy
²⁴ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁵ ICREA, 08010 Barcelona, Spain
²⁶ now at Ecole polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
²⁷ supported by INFN Padova  
²⁸ now at: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
²⁹ now at: Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Finland
³⁰ HITS, Schloss-Wolfsbrunnenweg 33, 69118 Heidelberg, Germany
³¹ UC Santa Barbara, CA 93106, Santa Barbara, USA

Received: 16 December 2011
Accepted: 23 January 2012

Abstract

We report on the detection of very-high energy (VHE, E > 100 GeV) γ-ray emission from NGC 1275, the central radio galaxy of the Perseus cluster of galaxies. The source has been detected by the MAGIC telescopes with a statistical significance of 6.6σ above 100 GeV in 46 h of stereo observations carried out between August 2010 and February 2011. The measured differential energy spectrum between 70 GeV and 500 GeV can be described by a power law with a steep spectral index of Γ =  −4.1 ± 0.7_stat ± 0.3_syst, and the average flux above 100 GeV is F_γ = (1.3 ± 0.2_stat ± 0.3_syst) × 10^-11   cm^-2   s^-1. These results, combined with the power-law spectrum measured in the first two years of observations by the Fermi-LAT above 100 MeV, with a spectral index of Γ ≃  − 2.1, strongly suggest the presence of a break or cut-off around tens of GeV in the NGC 1275 spectrum. The light curve of the source above 100 GeV does not show hints of variability on a month time scale. Finally, we report on the nondetection in the present data of the radio galaxy IC 310, previously discovered by the Fermi-LAT and MAGIC. The derived flux upper limit F^U.L._γ (>300 GeV) = 1.2 × 10^-12 cm^-2 s^-1 is a factor  ~ 3 lower than the mean flux measured by MAGIC between October 2009 and February 2010, thus confirming the year time-scale variability of the source at VHE.