Variability in Quasar Broad Absorption Line Outflows I. Trends in the Short-Term versus Long-Term Data

TL;DR

This study investigates the variability of quasar broad absorption lines over short and long timescales, revealing that variability depends on outflow velocity and BAL strength, with implications for the structure and location of the outflows.

Contribution

First systematic comparison of BAL variability over short and long timescales, showing velocity and strength dependence, and constraining absorber locations within 6 parsecs.

Findings

Long-term BAL variability is greater than short-term.

Higher velocity and weaker BALs are more variable.

Variability suggests absorbers are within 6 parsecs of the quasar.

Abstract

Broad absorption lines (BALs) in quasar spectra identify high velocity outflows that likely exist in all quasars and could play a major role in feedback to galaxy evolution. The variability of BALs can help us understand the structure, evolution, and basic physical properties of the outflows. Here we report on our first results from an ongoing BAL monitoring campaign of a sample of 24 luminous quasars at redshifts 1.2<z<2.9, focusing on C IV 1549 BAL variability in two different time intervals: 4 to 9 months (short-term) and 3.8 to 7.7 years (long-term) in the quasar rest-frame. We find that 39% (7/18) of the quasars varied in the short-term, whereas 65% (15/23) varied in the long-term, with a larger typical change in strength in the long-term data. The variability occurs typically in only portions of the BAL troughs. The components at higher outflow velocities are more likely to vary than those at lower velocities, and weaker BALs are more likely to vary than stronger BALs. The fractional change in BAL strength correlates inversely with the strength of the BAL feature, but does not correlate with the outflow velocity. Both the short-term and long-term data indicate the same trends. The observed behavior is most readily understood as a result of the movement of clouds across the continuum source. If the crossing speeds do not exceed the local Keplerian velocity, then the observed short-term variations imply that the absorbers are <6 pc from the central quasar.