MAGIC observations of the giant radio galaxy M 87 in a low-emission state between 2005 and 2007

¹ IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
² Universidad Complutense, 28040 Madrid, Spain
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
e-mail: fabrizio.tavecchio@brera.inaf.it
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
⁶ Università di Padova and INFN, 35131 Padova, Italy
⁷ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
e-mail: berger@astro.uni-wuerzburg.de
⁸ Depto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, 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 Universität Würzburg, 97074 Würzburg, Germany
¹⁵ Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
¹⁶ Università di Udine, and INFN Trieste, 33100 Udine, Italy
¹⁷ Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
¹⁸ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
¹⁹ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
e-mail: tterzic@phy.uniri.hr; dijana@phy.uniri.hr
²⁰ Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
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 : École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
²⁷ Supported by INFN Padova Italia
²⁸ Now a: 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 INAF/Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy

Received: 4 August 2011
Accepted: 13 June 2012

Abstract

Context. We present the results of a long M 87 monitoring campaign in very high energy γ-rays with the MAGIC-I Cherenkov telescope.

Aims. We aim to model the persistent non-thermal jet emission by monitoring and characterizing the very high energy γ-ray emission of M 87 during a low state.

Methods. A total of 150 h of data were taken between 2005 and 2007 with the single MAGIC-I telescope, out of which 128.6 h survived the data quality selection. We also collected data in the X-ray and Fermi-LAT bands from the literature (partially contemporaneous).

Results. No flaring activity was found during the campaign. The source was found to be in a persistent low-emission state, which was at a confidence level of 7σ. We present the spectrum between 100 GeV and 2 TeV, which is consistent with a simple power law with a photon index Γ = 2.21 ± 0.21 and a flux normalization at 300 GeV of (7.7 ± 1.3) × 10^-8 TeV^-1 s^-1 m^-2. The extrapolation of the MAGIC spectrum into the GeV energy range matches the previously published Fermi-LAT spectrum well, covering a combined energy range of four orders of magnitude with the same spectral index. We model the broad band energy spectrum with a spine layer model, which can satisfactorily describe our data.