Soft X-ray photoelectric absorption of high-z quasars has been known for two decades, but its physical origin, whether intrinsic to the quasar, or in the intergalactic medium (IGM) has mostly remained a mystery. We have constructed the largest sample to date of more than 60 high redshift quasars (z > 0.5) selected from the XMM-Newton archive based on high photon count criteria. We analyze the measured optical depth tau at 0.5 keV for the sample, in an attempt to find which physical parameters of the quasars drive the observed absorption, e.g., redshift, radio luminosity, radio loudness, or the X-ray luminosity. Clearly, high-z quasars are more absorbed than low-z quasars, and radio-loud quasars are more absorbed than radio quiet ones. We confront the quasar absorption dependence on redshift with a comparison sample of gamma ray bursts (GRBs). The mean 0.5 keV optical depths of both the quasars and the GRBs tend at high redshift to a similar value of tau ~ 0.4. Interestingly, such behavior is expected if the absorption arises from the diffuse IGM. However, quasar X-ray opacity at intermediate redshift (0.5 < z < 2) is generally too low, deeming the diffuse IGM scenario unlikely.
Followed by wine and cheese in Pupin 1402.