Tempestuous life beyond R₅₀₀: X-ray view on the Coma cluster with SRG/eROSITA

II. Shock and relic

¹ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
e-mail: churazov@mpa-garching.mpg.de
² Space Research Institute (IKI), Profsoyuznaya 84/32, Moscow 117997, Russia
³ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
⁴ Ioffe Institute, Politekhnicheskaya St. 26, Saint Petersburg 194021, Russia
⁵ ASC of P.N. Lebedev Physical Institute, Leninskiy Prospect 53, Moscow 119991, Russia

Received: 13 May 2022
Accepted: 24 November 2022

Abstract

This is the second paper in a series of studies of the Coma cluster using the SRG/eROSITA X-ray data obtained during the calibration and performance verification phase of the mission. Here, we focus on the region adjacent to the radio source 1253+275 (radio relic, RR, hereafter). We show that the X-ray surface brightness exhibits its steepest gradient at ∼79′ (∼2.2 Mpc ≈ R_200c), which is almost co-spatial to the outer edge of the RR. As in the case of several other relics, the Mach number of the shock derived from the X-ray surface brightness profile (M_X ≈ 1.9) appears to be lower than needed to explain the slope of the integrated radio spectrum in the diffusive shock acceleration model (M_R ≈ 3.5) if the magnetic field is uniform and the radiative losses are fast. However, the shock geometry is plausibly much more complicated than a spherical wedge centered on the cluster, given the non-trivial correlation between radio, X-ray, and SZ images. While the complicated shock geometry alone might cause a negative bias in M_X, we speculate on a few other possibilities that may affect the M_X − M_R relation, including the shock substructure that might be modified by the presence of non-thermal filaments stretching across the shock and the propagation of relativistic electrons along the non-thermal filaments with a strong magnetic field. We also discuss the “history” of the radio galaxy NGC 4789, which is located ahead of the relic in the context of the Coma-NGC 4839 merger scenario.