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

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.

💡 Research Summary

This paper presents a comprehensive analysis of deep NuSTAR observations (total exposure ≈ 270 ks, with 264 ks after cleaning) of the merging galaxy cluster ZWCL 1856.8+6616 at redshift z = 0.304, focusing on the relationship between its double radio relics and X‑ray shock fronts. Building on a pilot study that used only 30 ks of NuSTAR data (Tümer et al. 2024), the authors obtain sufficient photon statistics to map the intracluster medium (ICM) temperature structure across the field of view and to quantify shock strengths at the locations of the northern (NR) and southern (SR) relics.

The data reduction follows standard HEASoft v6.32 and nustardas v2.1.2 pipelines, with background modeled using nuskybgd. Exposure‑corrected images in the 3–8 keV (soft) and 8–15 keV (hard) bands are shown alongside a LOFAR 140 MHz radio map, illustrating the spatial coincidence of the relics with potential X‑ray surface‑brightness edges. Thirteen regions of interest (ROIs) are defined based on optical, radio, and X‑ray morphology: central, eastern, western, pre‑ and post‑shock zones for each relic, the relics themselves, and several point‑source regions identified from Chandra data.

Initial spectral fitting with XSPEC uses a single‑temperature APEC model for each diffuse ROI (redshift fixed at 0.304, metallicity at 0.3 Z⊙). The central region yields kT ≈ 5.5 keV; adding a second thermal component improves the fit, revealing a hotter component (kT ≈ 6 keV) and a faint, very cool component (kT ≈ 0.7 keV) that contributes only ~5 % of the total counts, suggesting it may be an artifact of residual cross‑contamination rather than a genuine physical component.

A critical challenge is NuSTAR’s moderate point‑spread function (≈ 1′ half‑power diameter, slightly energy‑dependent). Photons from one ROI spill into neighboring regions, producing “cross‑talk” that can bias temperature and normalization estimates, especially in low‑surface‑brightness outskirts. To address this, the authors employ the IDL‑based nucrossarf toolkit, which generates cross‑ARFs that explicitly model the contribution of each ROI’s emission to every other extraction region. Two nucrossarf runs are performed: a “Main Fit” with only thermal APEC components, and a second run that adds a power‑law component to the relic ROIs to search for inverse‑Compton (IC) emission. Point sources are modeled with fixed photon indices (derived from nuproducts) but free normalizations. All thermal components retain the same redshift and metallicity constraints.

After incorporating cross‑talk, the post‑shock regions (NR + post‑shock ROI, SR + post‑shock ROI) exhibit clear temperature jumps relative to their pre‑shock counterparts. Using the Rankine‑Hugoniot relation for an ideal mono‑atomic gas, the temperature ratio for the northern shock corresponds to a Mach number M = 3.90 + 1.64/‑0.85, significantly higher than the radio‑derived Mach number M = 2.5 ± 0.2 reported by Jones et al. (2021). The southern shock yields M = 2.36 + 0.58/‑0.46. These values place the northern shock among the strongest X‑ray‑detected shocks in galaxy clusters, exceeding the typical M ≲ 3 range expected from the high sound speeds in hot ICM.

The discrepancy between X‑ray and radio Mach numbers is interpreted as evidence that the northern radio relic is confined to a very narrow region on the sky, possibly representing a localized zone of enhanced particle acceleration efficiency. The authors suggest that while the radio emission traces the immediate shock front, the X‑ray temperature jump reflects a broader, higher‑Mach shock that may extend beyond the radio‑bright region.

A search for non‑thermal IC emission from the relics was conducted by adding a power‑law (photon index ≈ 2) to the relic spectra. No statistically significant IC component was detected, implying that the relativistic electron population does not produce a detectable hard X‑ray excess at the current depth, or that the magnetic field strength is sufficiently high to suppress IC relative to synchrotron emission.

The paper concludes that deep NuSTAR observations, combined with rigorous cross‑talk correction, enable robust measurements of shock strengths in merging clusters even when the PSF is modest. The results reinforce the physical connection between radio relics and merger‑driven shocks, while also highlighting that X‑ray and radio diagnostics can yield different Mach estimates due to geometry, projection, and acceleration efficiency variations. The methodology demonstrated here provides a template for future high‑energy studies of cluster outskirts, especially when complemented by higher‑resolution instruments (e.g., Chandra, XMM‑Newton) and upcoming missions such as Athena or Lynx, which will further constrain the thermal and non‑thermal components of the ICM.