Quasar Broad Absorption Line Variability on Multi-Year Time Scales

TL;DR

This study analyzes multi-year variability of quasar broad absorption lines, revealing complex changes over decades, and suggests that outflow geometry, rather than photoionization, primarily drives these variations.

Contribution

It provides the first multi-year analysis of CIV BAL variability in quasars, constraining BAL lifetimes and exploring the physical mechanisms behind variability.

Findings

BALs change in complex ways over 3-6 years

BAL lifetime is at least a few decades

Variation likely driven by outflow geometry changes

Abstract

We use quantitative metrics to characterize the variation of CIV 1549 A broad absorption lines (BALs) over 3--6 (rest-frame) years in a sample of 13 quasars at 1.7 <= z <= 2.8 and compare the results to previous studies of BAL variability on shorter time scales. The strong BALs in our study change in complex ways over 3--6 yr. Variation occurs in discrete regions which are only a few thousand km/s wide, and the distribution of the change in absorption equivalent width broadens over time. We constrain the typical CIV BAL lifetime to be at least a few decades. While we do not find evidence to support a scenario in which the variation is primarily driven by photoionization on multi-year time scales, there is some indication that the variation is produced by changes in outflow geometry. We do not observe significant changes in the BAL onset velocity, indicating that the absorber is either far from the source or is being continually replenished and is azimuthally symmetric. It is not possible in a human lifetime to expand the time scales in our study by more than a factor of a few using optical spectroscopy. However, the strong variation we have observed in some BALs indicates that future studies of large numbers of BAL QSOs will be valuable to constrain BAL lifetimes and the physics of variation.