A revision of the X-ray absorption nature of the BALQSOs

TL;DR

This study investigates the X-ray absorption properties of BALQSOs using XMM-Newton data, revealing that most are absorbed when modeled with ionized absorption, and finds a new correlation between X-ray absorption and UV BAL parameters.

Contribution

The paper provides the first comprehensive analysis of X-ray absorption in a large BALQSO sample, highlighting differences between LoBALs and HiBALs and establishing a new correlation with UV properties.

Findings

Most BALQSOs are absorbed when modeled with ionized gas.

LoBALs may be physically distinct from HiBALs.

X-ray absorption correlates with UV BAL strength at 98% confidence.

Abstract

Broad absorption line quasars (BALQSOs) are key objects for studying the structure and emission/absorption properties of AGN. However, despite their fundamental importance, the properties of BALQSOs are still not well understood. In order to investigate the X-ray nature of these sources, as well as the correlations between X-ray and rest-frame UV properties, we compile a large sample of 88 BALQSOs observed by XMM-Newton. We performed a full X-ray spectral analysis on a sample of 39 sources with higher X-ray spectral quality, and an approximate HR analysis on the remaining sources. Using available optical spectra, we calculate the BALnicity index and investigate the dependence between this optical parameter and different X-ray properties. Using the neutral absorption model, we found that 36% of our BALQSOs have NH < 5x10^21 cm^-2, lower than the expected X-ray absorption for such objects. However, when we used a physically-motivated model for the X-ray absorption in BALQSOs, i.e. ionized absorption, \sim 90% of the objects are absorbed. The absorption properties also suggest that LoBALs may be physically different objects from HiBALs. In addition, we report on a correlation between the ionized absorption column density and BAL parameters. There is evidence (at 98% level) that the amount of X-ray absorption is correlated with the strength of high-ionization UV absorption. This correlation, not previously reported, can be naturally understood in virtually all BALQSO models, as driven by the total amount of gas mass flowing towards the observer.