Astro Seminar - 10/Nov/2010

Speaker: Prof. Ehud Bakar, Technion

Title: On the soft X-ray opacity towards high redshift GRBs and QSOs and the diffused intergalactic medium

Abstract:

Observations with the Swift satellite of X-ray afterglows of gamma ray bursts (GRBs) reveal ubiquitous soft X-ray absorption. If one assumes the absorber is at the redshift of the burst the implied absorbing column density then strongly increases with redshift z. I will show, however, that the directly measured optical depth at a given photon energy tends to a constant at high z. Such an asymptotic optical depth is indeed expected, if at high redshift the foreground diffused intergalactic medium (IGM) dominates the absorption effect. To further test this hypothesis, we have analyzed the 12 highest S/N z > 2 quasar spectra from the XMM-Newton archive, which all turn out to be radio loud (RLQs). The quasar spectra reveal a similar absorption trend to that of the GRBs, but with somewhat lower opacity. The best S/N X-ray spectra of radio-quiet quasars (RQQs) at z > 2, on the other hand, have much lower S/N and apparently less absorption. If this result holds in a larger higher-quality sample, it may allude to an intrinsic absorber rather than to the IGM. This puzzling anomaly of RQQs will be solved only with high X-ray spectral resolution that will distinguish between intrinsic discrete spectral features, and the predominantly diffuse IGM that produces significant absorption, but with no discernible spectral features.