Deep observation of the NGC 1275 region with MAGIC: search of diffuse γ-ray emission from cosmic rays in the Perseus cluster

¹ ETH Zurich, 8093 Zurich, Switzerland
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, University of Split and University of Zagreb, 51000 Rijeka, Croatia
⁶ Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Salt Lake, Sector-1, 700064 Kolkata, India
⁷ Max-Planck-Institut für Physik, 80805 München, Germany
⁸ Universidad Complutense, 28040 Madrid, Spain
⁹ Inst. de Astrofísica de Canarias; Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁰ University of Łódź, 90236 Lodz, Poland
¹¹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹² Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra ( Barcelona), Spain
¹³ Universität Würzburg, 97074 Würzburg, Germany
¹⁴ Università di Padova and INFN, 35131 Padova, Italy
¹⁵ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁶ Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
¹⁷ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁸ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁹ Universitat de Barcelona,ICC, IEEC-UB, 08028 Barcelona, Spain
²⁰ Japanese MAGIC Consortium, ICRR, The University of Tokyo, Department of Physics and Hakubi Center, Kyoto University, Tokai University, The University of Tokushima, KEK, Japan
²¹ Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, Finland
²² Inst. for Nucl. Research and Nucl. Energy, 1784Sofia, Bulgaria
²³ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁴ ICREA and Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
²⁵ now at Centro Brasileiro de Pesquisas Físicas (CBPF/MCTI), R. Dr. Xavier Sigaud, 150 – Urca, RJ, 22290-180 Rio de Janeiro, Brazil
²⁶ now at NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA and Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁷ Humboldt University of Berlin, Institut für Physik Newtonstr. 15, 12489 Berlin, Germany
²⁸ Now at Ecole polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
²⁹ Now at Department of Physics & Astronomy, UC Riverside, CA 92521, USA
³⁰ Also at Japanese MAGIC Consortium  
³¹ Now at Finnish Centre for Astronomy with ESO (FINCA), 20740 Turku, Finland
³² Also at INAF-Trieste  
³³ Also at ISDC – Science Data Center for Astrophysics, 1290 Versoix ( Geneva), Switzerland
³⁴ Heidelberg Institute for Theoretical Studies, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
³⁵ Dark Cosmology Center, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
³⁶ GRAPPA Institute, University of Amsterdam, 1098 XH Amsterdam, The Netherlands

Received: 25 November 2015
Accepted: 4 February 2016

Abstract

Clusters of galaxies are expected to be reservoirs of cosmic rays (CRs) that should produce diffuse γ-ray emission due to their hadronic interactions with the intra-cluster medium. The nearby Perseus cool-core cluster, identified as the most promising target to search for such an emission, has been observed with the MAGIC telescopes at very-high energies (VHE, E ≳ 100 GeV) for a total of 253 h from 2009 to 2014. The active nuclei of NGC 1275, the central dominant galaxy of the cluster, and IC 310, lying at about 0.6° from the centre, have been detected as point-like VHE γ-ray emitters during the first phase of this campaign. We report an updated measurement of the NGC 1275 spectrum, which is described well by a power law with a photon index Γ = 3.6 ± 0.2_stat ± 0.2_syst between 90 GeV and 1200 GeV. We do not detect any diffuse γ-ray emission from the cluster and so set stringent constraints on its CR population. To bracket the uncertainties over the CR spatial and spectral distributions, we adopt different spatial templates and power-law spectral indexes α. For α = 2.2, the CR-to-thermal pressure within the cluster virial radius is constrained to be ≲ 1−2%, except if CRs can propagate out of the cluster core, generating a flatter radial distribution and releasing the CR-to-thermal pressure constraint to ≲ 20%. Assuming that the observed radio mini-halo of Perseus is generated by secondary electrons from CR hadronic interactions, we can derive lower limits on the central magnetic field, B₀, that depend on the CR distribution. For α = 2.2, B₀ ≳ 5−8 μG, which is below the ~25 μG inferred from Faraday rotation measurements, whereas for α ≲ 2.1, the hadronic interpretation of the diffuse radio emission contrasts with our γ-ray flux upper limits independently of the magnetic field strength.