X-ray evidence from NuSTAR for a Mach 3 shock in Merging Galaxy Cluster ZWCL 1856.8

TL;DR

This study uses deep NuSTAR X-ray observations to detect and analyze shock features in a merging galaxy cluster, revealing a very strong shock at the northern relic and exploring the relation between radio and X-ray shock signatures.

Contribution

First detailed spectral analysis of the merging galaxy cluster ZWCL 1856.8+6616 with NuSTAR, identifying one of the strongest X-ray detected shocks in a cluster merger.

Findings

Detected a shock with Mach number 3.90 at the northern relic.

Measured a Mach number of 2.36 at the southern shock.

No inverse Compton emission was detected at the relic sites.

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 (Tumer et al. 2024). 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-ARFs 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=3.90(+1.64,-0.85) at the Northern relic site, that is unprecedentedly larger than the radio counterpart; M=2.5+/-0.2 (Jones et al. 2021a), and we report Southern shock strength as M=2.36(+0.58,-0.46). 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.