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

Ayşegül Tümer, Christian T. Norseth, Daniel R. Wik

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.

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 (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.
Paper Structure (7 sections, 1 equation, 3 figures)

This paper contains 7 sections, 1 equation, 3 figures.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Data Analysis and Results
  4. Setting The Stage For nucrossarf: Selection of Regions of Interest and Designation of Initial Parameters
  5. Region Selection
  6. Assessment of Initial Fit Parameter Values
  7. Tackling the Cross-Talk: nucrossarf

Figures (3)

  • Figure 1: Background subtracted, exposure corrected, NuSTAR images of ZWCL 1856.8 smoothed with a Gaussian of r=5 pixels (1 pixel subtending 12$\hbox{$^{\prime\prime}$}$.3) in the soft ( left) and hard bands ( middle). Right: and LOFAR image at 140 MHz (resolution: 13$\hbox{$^{\prime\prime}$}$$\times$ 7$\hbox{$^{\prime\prime}$}$, RMS noise: 127 $\mu$Jy/beam).
  • Figure 2: Top Panel: Selected regions overlaid on background subtracted, exposure corrected images Bottom Panel: Region numbers.
  • Figure 3: Central region spectra showing NuSTAR fits using a single and two temperature models. Background is indicated with asterix symbols. For plotting purposes, adjacent bins are grouped until they have a significant detection at least as large as 8$\sigma$, with maximum 12 bins.