Chandra and XMM-Newton observations of A2256: cold fronts, merger shocks, and constraint on the IC emission

TL;DR

This study uses deep X-ray observations to analyze the complex merging activity in galaxy cluster A2256, identifying shock fronts, cold fronts, and constraining the magnetic field through inverse-Compton emission limits.

Contribution

The paper provides detailed X-ray mapping of A2256's merger dynamics, identifying multiple cold fronts and shock fronts, and sets new constraints on the magnetic field strength in the radio relic region.

Findings

Identification of five X-ray surface brightness edges, including cold fronts and shock fronts.

Measurement of Mach numbers for shock fronts near the radio relic.

Estimation of a lower limit on magnetic field strength in the cluster.

Abstract

We present the results of deep Chandra and XMM-Newton observations of a complex merging galaxy cluster Abell 2256 (A2256) that hosts a spectacular radio relic (RR). The temperature and metallicity maps show clear evidence of a merger between the western subcluster (SC) and the primary cluster (PC). We detect five X-ray surface brightness edges. Three of them near the cluster center are cold fronts (CFs): CF1 is associated with the infalling SC; CF2 is located in the east of the PC; and CF3 is to the west of the PC core. The other two edges at cluster outskirts are shock fronts (SFs): SF1 near the RR in the NW has Mach numbers derived from the temperature and the density jumps, respectively, of $M_T=1.62\pm0.12$ and $M_\rho=1.23\pm0.06$; SF2 in the SE has $M_T=1.54\pm0.05$ and $M_\rho=1.16\pm0.13$. In the region of the RR, there is no evidence for the correlation between X-ray and radio substructures, from which we estimate an upper limit for the inverse-Compton emission, and therefore set a lower limit on the magnetic field ($\sim$ 450 kpc from PC center) of $B>1.0\ \mu$G for a single power-law electron spectrum or $B>0.4\ \mu$G for a broken power-law electron spectrum. We propose a merger scenario including a PC, an SC, and a group. Our merger scenario accounts for the X-ray edges, diffuse radio features, and galaxy kinematics, as well as projection effects.