Evidence of a Hard Excess in NuSTAR Spectra of the Perseus Cluster’s Diffuse Intracluster Medium

As the brightest galaxy cluster in the X-ray sky, Perseus is an excellent target for studying the Intracluster Medium (ICM), but until recently, the bright AGN made studies of the diffuse emission near its center nearly impossible to accomplish with NuSTAR due to the extended wings of its PSF. The development of a new open source software package— nucrossarf—now allows the contribution from point and diffuse sources to be modeled across regions so that scattered light from the AGN can be properly accounted for. Using this technique, we present an analysis of the diffuse hard X-ray (3–25 keV) emission from the ICM using three archival NuSTAR observations of the Perseus cluster. We have characterized the temperature distribution of the ICM and find that the results are consistent with previous temperature maps produced by Chandra. Additionally, we find a ∼10% excess of emission above 20 keV that cannot be realistically described by purely thermal models. In order to better constrain the results, we have characterized the systematic uncertainty of the modeled AGN contribution to be 2.5%—based on similar analyses of bright AGN observations in the archive—and we find that the excess cannot be explained by scattered photons from the AGN. We discuss the potential origin and implications of this excess and present our analysis of the nucrossarf systematic uncertainty, which will be useful for future work.