Study of the GeV to TeV morphology of the γ Cygni SNR (G 78.2+2.1) with MAGIC and Fermi-LAT

Evidence for cosmic ray escape

¹ Inst. de Astrofísica de Canarias, E-38200 La Laguna, and Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁴ ETH Zurich, CH-8093 Zurich, Switzerland
⁵ Technische Universität Dortmund, D-44221 Dortmund, Germany
⁶ Croatian Consortium: University of Rijeka, Department of Physics, 51000 Rijeka ; University of Split – FESB, 21000 Split ; University of Zagreb – FER, 10000 Zagreb ; University of Osijek, 31000 Osijek ; Rudjer Boskovic Institute, 10000 Zagreb, Croatia
⁷ Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
⁸ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro (RJ), Brasil
⁹ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁰ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹¹ Università di Siena and INFN Pisa, 53100 Siena, Italy
¹² Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹³ Istituto Nazionale Fisica Nucleare (INFN), 00044 Frascati (Roma), Italy
¹⁴ Max-Planck-Institut für Physik, 80805 München, Germany
¹⁵ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra (Barcelona), Spain
¹⁶ Università di Padova and INFN, 35131 Padova, Italy
¹⁷ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
¹⁸ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
¹⁹ The Armenian Consortium: ICRANet-Armenia at NAS RA, A. Alikhanyan National Laboratory, Republic of Armenia
²⁰ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, E-28040 Madrid, Spain
²¹ Universität Würzburg, 97074 Würzburg, Germany
²² Finnish MAGIC Consortium: Finnish Centre of Astronomy with ESO (FINCA), University of Turku, 20014 Turku ; Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
²³ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁴ Japanese MAGIC Consortium: ICRR, The University of Tokyo, 277-8582 Chiba ; Department of Physics, Kyoto University, 606-8502 Kyoto ; Tokai University, 259-1292 Kanagawa ; RIKEN, 351-0198 Saitama, Japan
²⁵ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²⁶ Sapienza Università di Roma and INFN Roma, 00185 Rome, Italy
²⁷ INAF, Osservatorio Astrofisico di Arcetri, 50125 Firenze, Italy
²⁸ Institute for Astro- and Particle Physics, University of Innsbruck, A-6020 Innsbruck, Austria
²⁹ Port d’Informació Científica (PIC) E-08193 Bellaterra (Barcelona), Spain
³⁰ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
³¹ INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Italy

Received: 25 June 2020
Accepted: 13 October 2020

Abstract

Context. Diffusive shock acceleration (DSA) is the most promising mechanism that accelerates Galactic cosmic rays (CRs) in the shocks of supernova remnants (SNRs). It is based on particles scattering caused by turbulence ahead and behind the shock. The turbulence upstream is supposedly generated by the CRs, but this process is not well understood. The dominant mechanism may depend on the evolutionary state of the shock and can be studied via the CRs escaping upstream into the interstellar medium (ISM).

Aims. Previous observations of the γ Cygni SNR showed a difference in morphology between GeV and TeV energies. Since this SNR has the right age and is at the evolutionary stage for a significant fraction of CRs to escape, our aim is to understand γ-ray emission in the vicinity of the γ Cygni SNR.

Methods. We observed the region of the γ Cygni SNR with the MAGIC Imaging Atmospheric Cherenkov telescopes between 2015 May and 2017 September recording 87 h of good-quality data. Additionally, we analysed Fermi-LAT data to study the energy dependence of the morphology as well as the energy spectrum in the GeV to TeV range. The energy spectra and morphology were compared against theoretical predictions, which include a detailed derivation of the CR escape process and their γ-ray generation.

Results. The MAGIC and Fermi-LAT data allowed us to identify three emission regions that can be associated with the SNR and that dominate at different energies. Our hadronic emission model accounts well for the morphology and energy spectrum of all source components. It constrains the time-dependence of the maximum energy of the CRs at the shock, the time-dependence of the level of turbulence, and the diffusion coefficient immediately outside the SNR shock. While in agreement with the standard picture of DSA, the time-dependence of the maximum energy was found to be steeper than predicted, and the level of turbulence was found to change over the lifetime of the SNR.