X-ray Absorption of High Redshift Quasars

TL;DR

This study analyzes X-ray absorption in high-redshift quasars, finding that absorption increases with redshift but is unlikely due to diffuse intergalactic medium, suggesting local or complex origins.

Contribution

It provides the largest sample to date of high-z quasars with measured X-ray absorption, and investigates the physical drivers behind their absorption properties.

Findings

43% of quasars show significant absorption

Absorption correlates weakly with redshift and radio luminosity

High-z quasar absorption is inconsistent with diffuse IGM origin

Abstract

Soft X-ray photoelectric absorption of high-z quasars has been known for two decades, but has no unambiguous astro-physical context. We construct the largest sample to date of 58 high redshift quasars (z > 0.45) selected from the XMM-Newton archive based on a high photon count criterion (> 1800). We measure the optical depth tau at 0.5 keV, and find that 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 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 burst (GRB) X-ray afterglows. Although the z > 2 quasar opacity is consistent with diffuse IGM absorption, many intermediate z (0.45 < z < 2) quasars are not sufficiently absorbed for this scenario, and are appreciably less absorbed than GRBs. Only 10/37 quasars at z < 2 are absorbed, and only 5/30 radio quiet quasars are absorbed. We find a weak correlation between tau and z, and even a weaker correlation between tau and radio luminosity, which leads to the conclusion that although a diffuse IGM origin for the quasar absorption is unlikely, optical depth does seem to increase with redshift, roughly as (1+z)^{2.2+-0.6}, tending at high-z to tau ~ 0.4, similar to the high-z GRB values. This result can be explained by an ionized and clumpy IGM at z < 2, and a cold, diffuse IGM at higher redshift. If, conversely, ascribed to local absorption at the quasar, and owing to the steep L_x ~ (1+z)^{7.1+-0.5} correlation in the present sample, the host column density scales as N_H ~ L_x^{0.7+-0.1}.