X-Ray Evidence from NuSTAR for a Mach 3 Shock in Merging Galaxy Cluster ZWCL 1856.8

Abstract We present spectral analysis results of deeper (270 ks) NuSTAR observations of the merging galaxy cluster system ZWCL1856.8+6616 at redshift z = 0.304 following a pilot study using shallower (30 ks) NuSTAR data. The cluster hosts a double radio relic, pointing to a similar mass head-on collision at/near the plane of sky. We aim to find the relation between radio and X-ray shock features. Using data from both focal plane modules of NuSTAR, we study the temperature structure across the field of view and report on the X-ray detected shock strength at the relic sites. We generate nominal and cross–ancillary response files with nucrossarf to disentangle photon cross-contamination within regions of interest due to the moderate point-spread function of NuSTAR. Here we report one of the strongest X-ray detected shocks in a galaxy cluster merger with M = 3.9 0 − 0.85 + 1.64 at the northern relic site, which is unprecedently larger than the radio counterpart, M = 2.5 ± 0.2, and we report the southern shock strength as M = 2.3 6 − 0.46 + 0.58 . We argue that since the northern relic (or radio shock) is confined in a very small region in the sky, particle acceleration is more efficient and is likely to grow in the post-shock regions. In addition, we search for inverse Compton (IC) emission at the radio relic sites; however, an IC component was not detected.