X-ray selected broad absorption line quasars in SDSS-V: BALs and non-BALs span the same range of X-ray properties

TL;DR

This study analyzes X-ray selected quasars to compare the properties of BAL and non-BAL quasars, finding similar X-ray characteristics and suggesting BAL gas is located further from the X-ray source, challenging previous notions of intrinsic X-ray weakness.

Contribution

It provides the first comprehensive comparison of BAL and non-BAL quasars within an X-ray selected sample, revealing similar X-ray properties and spatial distribution of the absorbing gas.

Findings

BAL fraction in X-ray selected quasars is ~6%, similar to optical samples.

No significant difference in column densities between BAL and non-BAL quasars.

BAL gas likely located further from the X-ray corona than the emission region.

Abstract

Broad absorption line (BAL) quasars are often considered X-ray weak relative to their optical/UV luminosity, whether intrinsically (i.e., the coronal emission is fainter) or due to large column densities of absorbing material. The SDSS-V is providing optical spectroscopy for samples of quasar candidates identified by eROSITA as well as Chandra, XMM or Swift, making the resulting datasets ideal for characterising the BAL quasar population within an X-ray selected sample. We use the Balnicity Index (BI) to identify the BAL quasars based on absorption of the CIV $\lambda\,1549$ emission line in the optical spectra, finding 143 BAL quasars in our sample of 2317 X-ray selected quasars within $1.5\le z \le3.5$. This observed BAL fraction of $\approx$ 6 per cent is comparable to that found in optically selected samples. We also identify absorption systems via the Absorption Index (AI) which includes mini-BALs and NALs, finding 954 quasars with AI $>0$. We consider the CIV emission space (equivalent width vs. blueshift) to study the BAL outflows within the context of the radiatively driven accretion disc-wind model. X-ray selection excludes the highest outflow velocities in emission but includes the full range of absorption velocities which we suggest is consistent with the BAL gas being located further from the X-ray corona than the emitting gas. We observe both X-ray weak and X-ray strong BALs (via the optical-to-X-ray spectral slope, $\alpha_\text{ox}$) and detect little evidence for differing column densities between the BAL and non-BAL quasars, suggesting the BALs and non-BALs have the same shielding gas and intrinsic X-ray emission.