Evidence for Photoionization Driven Broad Absorption Line Variability

TL;DR

This study analyzes quasar broad absorption line variability, showing it is mainly driven by changes in gas ionization responding to continuum fluctuations, with coordinated variations observed among different ions and emission lines.

Contribution

It provides evidence that absorption line variability is primarily caused by ionization changes due to continuum variations, supported by multi-epoch spectroscopic data.

Findings

Absorption line variations are highly coordinated among different ions.

Line equivalent widths correlate with continuum brightness changes.

Emergence/disappearance of absorption components relate to continuum dimming/brightening.

Abstract

We present a qualitative analysis of the variability of quasar broad absorption lines using the large multi-epoch spectroscopic dataset of the Sloan Digital Sky Survey Data Release 10. We confirm that variations of absorption lines are highly coordinated among different components of the same ion or the same absorption component of different ions for C IV, Si IV and N V. Furthermore, we show that the equivalent widths of the lines decrease or increase statistically when the continuum brightens or dims. This is further supported by the synchronized variations of emission and absorption line equivalent width, when the well established intrinsic Baldwin effect for emission lines is taken into account. We find that the emergence of an absorption component is usually accompanying with dimming of the continuum while the disappearance of an absorption line component with brightening of the continuum. This suggests that the emergence or disappearance of a C IV absorption component is only the extreme case, when the ionic column density is very sensitive to continuum variations or the continuum variability amplitude is larger. These results support the idea that absorption line variability is driven mainly by changes in the gas ionization in response to continuum variations, that the line-absorbing gas is highly ionized, and in some extreme cases, too highly ionized to be detected in UV absorption lines. Due to uncertainties in the spectroscopic flux calibration, we cannot quantify the fraction of quasars with asynchronized continuum and absorption line variations.