Revisiting the Soft X-Ray Excess Emission in Clusters of Galaxies Observed with XMM-Newton

Abstract

We analyze XMM-Newton galaxy clusters Abell 1795, Abell S1101, Abell 1835, and MKW 3s in order to test whether their soft X-ray excess emission in the 0.2-0.5 keV band as reported by Kaastra et al. maintains after application of current knowledge of the XMM-Newton background and calibration. In this context, we examine the claim (Bregman & Lloyd-Davies) that the XMM-Newton sub-Galactic H I column density, and the accompanying soft excess continuum emission in the 0.2-0.5 keV band, is an artifact of an incorrect background subtraction. We show that since the cluster regions under scrutiny are within 500 kpc of the bright cluster center, the X-ray background level is negligible compared to the cluster emission level. Thus, at least in the central 500 kpc regions of the studied clusters, the reported soft excess is not a background artifact. We also study the possibility that the incomplete calibration information in the early phase of the XMM-Newton mission resulted in sub-Galactic H I column density, which was interpreted with the presence of a soft excess emitter. Our reanalysis of the four XMM-Newton observations with the MOS instruments yields evidence for the soft excess in all clusters. However, using the PN instrument, the best-fit H I column densities in Abell 1795, Abell 1835, and MKW 3s are at odds with the 2003 analysis of Kaastra et al. and in general agreement with those measured from 21 cm data in the direction of the clusters (Dickey & Lockman). Abell S1101 continues to feature a sub-Galactic NH also with the PN instrument, indicative of soft excess emission in both EPIC detectors. These differences are compatible with the current level of uncertainty in the calibration of both instruments.