Constraining cosmic rays and magnetic fields in the Perseus galaxy cluster with TeV observations 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
⁷ Università di Padova and INFN, 35131 Padova, Italy
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 Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
¹² Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹³ ETH Zurich, 8093 Zurich, Switzerland
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
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  
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: 22 November 2011
Accepted: 6 March 2012

Abstract

Galaxy clusters are being assembled today in the most energetic phase of hierarchical structure formation which manifests itself in powerful shocks that contribute to a substantial energy density of cosmic rays (CRs). Hence, clusters are expected to be luminous gamma-ray emitters since they also act as energy reservoirs for additional CR sources, such as active galactic nuclei and supernova-driven galactic winds. To detect the gamma-ray emission from CR interactions with the ambient cluster gas, we conducted the deepest to date observational campaign targeting a galaxy cluster at very high-energy gamma-rays and observed the Perseus cluster with the MAGIC Cherenkov telescopes for a total of  ~85 h of effective observing time. This campaign resulted in the detection of the central radio galaxy NGC 1275 at energies E > 100 GeV with a very steep energy spectrum. Here, we restrict our analysis to energies E > 630 GeV and detect no significant gamma-ray excess. This constrains the average CR-to-thermal pressure ratio to be  ≲1–2%, depending on assumptions and the model for CR emission. Comparing these gamma-ray upper limits to models inferred from cosmological cluster simulations that include CRs constrains the maximum CR acceleration efficiency at structure formation shocks to be  <50%.Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions. Finally, we derive lower limits on the magnetic field distribution assuming that the Perseus radio mini-halo is generated by secondary electrons/positrons that are created in hadronic CR interactions: assuming a spectrum of E^-2.2 around TeV energies as implied by cluster simulations, we limit the central magnetic field to be  >4–9 μG, depending on the rate of decline of the magnetic field strength toward larger radii. This range is well below field strengths inferred from Faraday rotation measurements in cool cores. Hence, the hadronic model remains a plausible explanation of the Perseus radio mini-halo.