Redshift evolution of X-ray spectral index of quasars observed by XMM-NEWTON/SDSS

Abstract

We study the X-ray spectral evolution of quasars with red-shift using a sample of 1280 quasars constructed by cross-referencing the XMM-Newton 3XMM−DR7 and the optical SDSS-DR14 catalogues. The sources were selected to have a minimum detected X-ray counts >600 and which were not affected by high background. The sample redshift distribution ranges from 0-4.9 and peaks around 1. For each source, the XMM-Newton EPIC_PN spectra were generated and fitted with a power-law model with Galactic absorption. Consistent with some earlier results we find a strong correlation between the photon index and redshift. We explicitly test the hypothesis that this is due to the X-ray soft excess and reflection bump being red-shifted away from the XMM band, by adding a black body and reflection component and find that the correlation is no longer significant. Thus, for the first time, we show that there is no evidence for quasar spectral evolution and the correlation between index and red-shift is indeed due to the soft excess and reflection component.