Cosmic-ray acceleration and escape from supernova remnant W44 as probed by Fermi-LAT and MAGIC

¹ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
² ETH Zürich, 8093 Zürich, Switzerland
³ Università di Siena and INFN Pisa, 53100 Siena, Italy
⁴ Instituto de Astrofísica de Canarias and Dpto. de Astrofísica, Universidad de La Laguna, 38200, La Laguna, Tenerife, Spain
⁵ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
⁶ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁸ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁹ Max-Planck-Institut für Physik, 85748 Garching, Germany
¹⁰ Università di Padova and INFN, 35131 Padova, Italy
¹¹ Croatian MAGIC Group: University of Zagreb, Faculty of Electrical Engineering and Computing (FER), 10000 Zagreb, Croatia
¹² IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹³ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro (RJ), Brazil
¹⁴ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹⁵ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁶ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra (Barcelona), Spain
¹⁷ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁸ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
¹⁹ INFN MAGIC Group: INFN Sezione di Bari and Dipartimento Interateneo di Fisica dell’Università e del Politecnico di Bari, 70125 Bari, Italy
²⁰ Armenian MAGIC Group: A. Alikhanyan National Science Laboratory, 0036 Yerevan, Armenia
²¹ Department for Physics and Technology, University of Bergen, Norway
²² INFN MAGIC Group: INFN Sezione di Torino and Università degli Studi di Torino, 10125 Torino, Italy
²³ INFN MAGIC Group: INFN Sezione di Catania and Dipartimento di Fisica e Astronomia, University of Catania, 95123 Catania, Italy
²⁴ Croatian MAGIC Group: University of Rijeka, Faculty of Physics, 51000 Rijeka, Croatia
²⁵ Universität Würzburg, 97074 Würzburg, Germany
²⁶ Technische Universität Dortmund, 44221 Dortmund, Germany
²⁷ University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
²⁸ Japanese MAGIC Group: Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 7398526 Hiroshima, Japan
²⁹ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
³⁰ Armenian MAGIC Group: ICRANet-Armenia, 0019 Yerevan, Armenia
³¹ Croatian MAGIC Group: University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture (FESB), 21000 Split, Croatia
³² Croatian MAGIC Group: Josip Juraj Strossmayer University of Osijek, Department of Physics, 31000 Osijek, Croatia
³³ Finnish MAGIC Group: Finnish Centre for Astronomy with ESO, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³⁴ Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka, 259-1292 Kanagawa, Japan
³⁵ Saha Institute of Nuclear Physics, A CI of Homi Bhabha National Institute, Kolkata 700064, West Bengal, India
³⁶ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
³⁷ Japanese MAGIC Group: Department of Physics, Yamagata University, Yamagata 990-8560, Japan
³⁸ Finnish MAGIC Group: Space Physics and Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
³⁹ Japanese MAGIC Group: Chiba University, ICEHAP, 263-8522 Chiba, Japan
⁴⁰ Japanese MAGIC Group: Institute for Space-Earth Environmental Research and Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, 464-6801 Nagoya, Japan
⁴¹ Japanese MAGIC Group: Department of Physics, Kyoto University, 606-8502 Kyoto, Japan
⁴² INFN MAGIC Group: INFN Roma Tor Vergata, 00133 Roma, Italy
⁴³ Japanese MAGIC Group: Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
⁴⁴ also at International Center for Relativistic Astrophysics (ICRA), Rome, Italy
⁴⁵ also at Port d’Informació Científica (PIC), E-08193 Bellaterra (Barcelona), Spain
⁴⁶ also at Institute for Astro- and Particle Physics, University of Innsbruck, 6020 Innsbruck, Austria
⁴⁷ also at Department of Physics, University of Oslo, Oslo, Norway
⁴⁸ also at Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁴⁹ Max-Planck-Institut für Physik, 85748 Garching, Germany
⁵⁰ also at INAF Padova, Italy
⁵¹ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁵² INFN Sezione di Bari, 70125 Bari, Italy
⁵³ INFN Sezione di Bari and Dipartimento Interateneo di Fisica dell’Università e del Politecnico di Bari, 70125 Bari, Italy
⁵⁴ INAF, Osservatorio Astrofisico di Arcetri, 50125 Firenze, Italy
⁵⁵ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland

^★ Corresponding authors; contact.magic@mpp.mpg.de

Received: 26 February 2024
Accepted: 29 November 2024

Abstract

Context. The supernova remnant (SNR) W44 and its surroundings are a prime target for studying the acceleration of cosmic rays (CRs). Several previous studies established an extended gamma-ray emission that is set apart from the radio shell of W44. This emission is thought to originate from escaped high-energy CRs that interact with a surrounding dense molecular cloud complex.

Aims. We present a detailed analysis of Fermi-LAT data with an emphasis on the spatial and spectral properties of W44 and its surroundings. We also report the results of the observations performed with the MAGIC telescopes of the northwestern region of W44. Finally, we present an interpretation model to explain the gamma-ray emission of the SNR and its surroundings.

Methods. We first performed a detailed spatial analysis of 12 years of Fermi-LAT data at energies above 1 GeV, in order to exploit the better angular resolution, while we set a threshold of 100 MeV for the spectral analysis. We performed a likelihood analysis of 174 hours of MAGIC data above 130 GeV using the spatial information obtained with Fermi-LAT.

Results. The combined spectra of Fermi-LAT and MAGIC, extending from 100 MeV to several TeV, were used to derive constraints on the escape of CRs. Using a time-dependent model to describe the particle acceleration and escape from the SNR, we show that the maximum energy of the accelerated particles has to be ≃40 GeV. However, our gamma-ray data suggest that a small number of lower-energy particles also needs to escape. We propose a novel model, the broken-shock scenario, to account for this effect and explain the gamma-ray emission.