X-Ray Insights Into the Physics of Mini-BAL Quasar Outflows

TL;DR

This study investigates the X-ray and UV properties of quasars with different CIV absorption line widths, revealing insights into the physical relationship and acceleration mechanisms of quasar outflows.

Contribution

It provides the first comparative analysis of X-ray characteristics of BAL and mini-BAL quasars, proposing mini-BALs as potential precursors to BALs influenced by X-ray shielding.

Findings

Mini-BALs have intermediate X-ray brightness and spectral slopes between BAL and non-BAL QSOs.

Some mini-BALs exhibit high outflow velocities without strong X-ray absorption.

X-ray bright mini-BALs may be in a high flux state or have unresolved companions.

Abstract

We examine the UV and X-ray properties of 256 radio-quiet SDSS quasars (QSOs) observed in X-rays with Chandra and/or XMM-Newton in order to study the relationship between QSOs with broad CIV absorption lines (BALs; width >2000 km/s) and those with CIV mini-BALs (here defined to have widths of 1000--2000 km/s). Our sample includes 42 BAL and 48 mini-BAL QSOs. The relative X-ray brightness and hard spectral slopes of the mini-BAL population are, on average, intermediate between those of BAL and non-BAL QSOs, as might be expected if narrower and broader absorption line outflows are physically related. However, a significant population of mini-BALs has outflow velocities higher than would be expected for BAL QSOs of the same relative X-ray brightness. Consistenly strong X-ray absorption is apparently not required to accelerate at least some mini-BALs to high outflow velocities. Assuming the mini-BAL features are correctly attributed to intrinsic CIV absorption, we suggest that their observed properties may be explained if mini-BALs are "seeds" which can be accelerated to form BALs when sufficient X-ray shielding is present. We also examine several QSOs with broad CIV absorption that have been recently reported to be unusually X-ray bright. Such cases are frequently mini-BAL QSOs, which as a population are generally brighter in X-rays than BAL QSOs. Pointed XMM-Newton observations also suggest that these sources (or unresolved neighbors) may have been previously observed in a high flux state.