ROSAT observations of the soft X-ray background and of the cluster soft excess emission in the Hercules supercluster

摘要

Recent observations with the XMM-Newton satellite confirmed the existence ofthe soft excess phenomenon in galaxy clusters, earlier discovered in severalEUVE, ROSAT and BeppoSAX observations. Among the clusters for which XMM hasreported detection of soft excess emission are MKW03s and A2052, two clustersin the Hercules concentration. The Hercules supercluster lies along the southern extension of the NorthPolar Spur, a region of bright soft X-ray emission clearly visible in ROSATAll-Sky Survey images. We analyze 11 pointed ROSAT PSPC observations toward 3clusters in the Hercule concentration, MKW03s, A2052 and A2063, and 8neighboring fields in order to investigate the soft X-ray emission in thatregion of the sky. We find that the soft X-ray emission varies by a factor offew on scales of few degrees, rendering the background subtraction a complextask. If the Noth Polar Spur emission is of local origin, we find that onlyA2052 and A2063 have evidence of cluster soft excess emission, and that theOVII emission lines detected in XMM observations of A2052 and MKW03s are notassociated with the cluster. If part or all of the North Polar Spur soft X-rayenhancement is of extragalactic nature, the three clusters feature strong softexcess emission, and the OVII emission lines observed by XMM are genuinelyassociated with the clusters. We interpret the soft excess emission with the presence of warm gas, eitherintermixed with the hot intra-cluster medium or in filamentary structureslocated around the clusters, and estimate that the warm gas is approximately asmassive as the hot intra-cluster medium.