Possible non-thermal origin of the hard X-ray emission in the merging galaxy cluster SPT-CL J2031-4037

TL;DR

This study analyzes NuSTAR X-ray data of the galaxy cluster SPT-CL J2031-4037, suggesting a possible non-thermal origin for its hard X-ray emission, but cannot definitively exclude a thermal explanation.

Contribution

First detailed NuSTAR analysis of this cluster's hard X-ray emission, exploring its non-thermal versus thermal origins with implications for cluster magnetic fields.

Findings

Possible non-thermal component in hard X-ray emission

Estimated non-thermal flux of approximately 4 x 10^{-12} erg s^{-1} cm^{-2}

Magnetic field strength estimated between 0.1-0.2 μG

Abstract

Non-thermal emission from clusters of galaxies at the high-energy X-ray regime has been searched with various instruments, but the detection significance of this emission has yet been found to be either marginal or controversial. Taking advantage of NuSTAR's unique capability to focus X-rays in the hard energy band, we present a detailed analysis of 238 ks NuSTAR observations of the merging galaxy cluster SPT-CL J2031-4037, searching for non-thermal inverse Compton emission. Our spectral analysis of SPT-CL J2031-4037 shows a possibility that the hard X-ray emission of the cluster can be described by a non-thermal component, though we cannot completely rule out a purely thermal origin for this hard emission. Including the statistical and systematic uncertainties, our best model fit yields a 20-80 keV non-thermal flux of $3.93_{-1.10}^{+1.24} \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$. The estimated non-thermal flux is comparable to those found in other galaxy clusters using NuSTAR and other X-ray instruments. Using this non-thermal flux with the existing radio data of the cluster, we estimate a volume-averaged magnetic field strength in the range of around 0.1-0.2 $\mu$G.