NuSTAR Observations of Abell 2163: Constraints on Non-thermal Emission

Abstract: Since the first non-thermal reports of inverse Compton (IC) emission from the intracluster medium (ICM) of galaxy clusters at hard X-ray energies, we have yet to unambiguously confirm IC emission in observations with newer facilities. RXTE detected IC emission in one of the hottest known clusters, Abell 2163 (A2163), a massive merging cluster with a giant radio halo--the presumed source of relativistic electrons IC scattering CMB photons to X-ray energies. The cluster's redshift (z~0.2) allows its thermal and non-thermal radio emission to fit NuSTARS's FOV, permitting a deep observation capable of confirming or ruling out the RXTE report. The IC flux provides constraints on the average magnetic field strength in a cluster. To determine the global diffuse IC emission in A2163, we fit its global NuSTAR spectrum with four models: single (1T) and two-temperature (2T), 1T+power law component (T+IC), and multi-temperature+power law (9T+IC). Each represent different characterizations of the thermal ICM emission, with power law components added to represent IC emission. We find the 3-30 keV spectrum can be described by purely thermal emission, with a global average temperature of kT = (11.8 $\pm$ 0.2) keV. The IC flux is constrained to $<$ $4.0$ $\times$ $10^{-12}$ $erg$ $s^{-1}$ $cm^{-2}$ using the 1T+IC model and $<$ $1.6$ $\times$ $10^{-12}$ $erg$ $s^{-1}$ $cm^{-2}$ with the more physical 9T+IC model, both to 90% confidence levels. Combining these limits with 1.4 GHz diffuse radio data from the VLA, we find the average magnetic field strength to be $>$ $0.22$$\mu$$G$ and $>$ $0.35$$\mu$$G$, respectively, providing the strongest constraints on these values in A2163 to date.