MAGIC observations of the diffuse γ-ray emission in the vicinity of the Galactic center^⋆

¹ Inst. de Astrofísica de Canarias, 38200 La Laguna, Spain
² Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁴ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁵ ETH Zurich, 8093 Zurich, Switzerland
⁶ Technische Universität Dortmund, 44221 Dortmund, Germany
⁷ 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
⁸ Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
⁹ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro, RJ, Brasil
¹⁰ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹¹ University of Łódź, Department of Astrophysics, 90236 Łódź, Poland
¹² Università di Siena and INFN Pisa, 53100 Siena, Italy
¹³ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹⁴ Istituto Nazionale Fisica Nucleare (INFN), 00044 Frascati, Roma, Italy
¹⁵ Max-Planck-Institut für Physik, 80805 München, Germany
¹⁶ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Barcelona, Spain
¹⁷ Università di Padova and INFN, 35131 Padova, Italy
¹⁸ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
¹⁹ ICRANet-Armenia at NAS RA, 0019 Yerevan, Armenia
²⁰ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 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, Finland; 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, Japan; Department of Physics, Kyoto University, 606-8502 Kyoto, Japan; Tokai University, 259-1292 Kanagawa, Japan; RIKEN, 351-0198 Saitama, Japan
²⁵ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²⁶ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
²⁷ Port d’Informació Científica (PIC), 08193 Bellaterra, Barcelona, Spain
²⁸ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
²⁹ INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Bologna, Italy

Received: 11 October 2019
Accepted: 24 May 2020

Abstract

Aims. In the presence of a sufficient amount of target material, γ-rays can be used as a tracer in the search for sources of Galactic cosmic rays (CRs). Here we present deep observations of the Galactic center (GC) region with the MAGIC telescopes and use them to infer the underlying CR distribution and to study the alleged PeV proton accelerator at the center of our Galaxy.

Methods. We used data from ≈100 h observations of the GC region conducted with the MAGIC telescopes over five years (from 2012 to 2017). Those were collected at high zenith angles (58−70 deg), leading to a larger energy threshold, but also an increased effective collection area compared to low zenith observations. Using recently developed software tools, we derived the instrument response and background models required for extracting the diffuse emission in the region. We used existing measurements of the gas distribution in the GC region to derive the underlying distribution of CRs. We present a discussion of the associated biases and limitations of such an approach.

Results. We obtain a significant detection for all four model components used to fit our data (Sgr A*, “Arc”, G0.9+0.1, and an extended component for the Galactic Ridge). We observe no significant difference between the γ-ray spectra of the immediate GC surroundings, which we model as a point source (Sgr A*) and the Galactic Ridge. The latter can be described as a power-law with index 2 and an exponential cut-off at around 20 TeV with the significance of the cut-off being only 2σ. The derived cosmic-ray profile hints to a peak at the GC position and with a measured profile index of 1.2 ± 0.3 is consistent with the 1/r radial distance scaling law, which supports the hypothesis of a CR accelerator at the GC. We argue that the measurements of this profile are presently limited by our knowledge of the gas distribution in the GC vicinity.