|M.Sc Student||Eitan Assaf|
|Subject||X-ray Absorption of High Redshift Quasars|
|Department||Department of Physics||Supervisor||Professor Ehud Behar|
|Full Thesis text|
Soft X-ray photoelectric absorption of high-z quasars has been known for two decades, but has no unambiguous astrophysical context. We construct the largest sample to date of 58 high redshift quasars z > 0.45 selected from the XMM-Newton archive based on high photon count criteria N > 1800. We present results for the directly measured optical depth τ at 0.5 keV for our sample, when 43% of the quasars show significant absorption. We aim to find which physical parameters of the quasars drive their observed absorption, e.g., redshift, radio luminosity, radio loudness, or the X-ray luminosity. We also compare the absorption behavior with redshift with the pattern expected if the diffuse intergalactic medium (IGM) is responsible for the observed absorption, and with a comparison sample of gamma ray bursts (GRBs). We find that although the z > 2 quasar opacity is consistent with IGM absorption, absorption of intermediate z (0.45 < z < 2) quasars is for the most part too low for this scenario, and appreciably lower than the opacity towards GRBs. Generally, z < 2 and radio quiet objects show no significant absorption, and provide only upper limits for the optical depth, when only 10/37 quasars at z < 2 are absorbed, and only 5/30 radio quiet quasars are absorbed. We use censored statistics methods to seek correlations between the measured parameters. We find only a weak correlation between τ and z, and even a weaker correlation between τ and radio luminosity, which leads to the conclusion that although a simple IGM origin for the quasar absorption is unlikely, optical depth does seem to increase with redshift, roughly as (1z)2.2?0.6, tending at high-z to τ ≈ 0.4, similar to the high-z GRB values. If ascribed to local absorption at the quasar, this implies a column density trend of NH ∝ Lx0.7?0.1.