Monitoring the Variability of Intrinsic Absorption Lines in Quasar Spectra

TL;DR

This study monitored intrinsic narrow and mini-broad absorption lines in quasars over several years, revealing variability in mini-BALs linked to ionization changes, while NALs remained stable, providing insights into the physical conditions and locations of the absorbing gas.

Contribution

First long-term variability study comparing NALs and mini-BALs in quasars, linking observed changes to ionization and coverage effects, and estimating absorber densities and distances.

Findings

Mini-BALs show significant variability in equivalent widths over years.

NALs exhibit no detectable variability during the monitoring period.

Variability in mini-BALs suggests gas densities of 10^3-10^5 cm^-3 and distances of a few kpc from the quasar core.

Abstract

We have monitored 12 intrinsic narrow absorption lines (NALs) in five quasars and seven mini-broad absorption lines (mini-BALs) in six quasars for a period of 4-12 years (1-3.5 years in the quasar rest-frame). We present the observational data and the conclusions that follow immediately from them, as a prelude to a more detailed analysis. We found clear variability in the equivalent widths (EWs) of the mini-BAL systems but no easily discernible changes in their profiles. We did not detect any variability in the NAL systems nor in narrow components that are often located at the center of mini-BAL profiles. Variations in mini-BAL EWs are larger at longer time intervals, reminiscent of the trend seen in variable broad absorption lines. If we assume that the observed variations result from changes in the ionization state of the mini-BAL gas, we infer lower limits to the gas density $\sim$ 10$^3$-10$^5$ cm$^{-3}$ and upper limits on the distance of the absorbers from the central engine of order a few kpc. Motivated by the observed variability properties, we suggest that mini-BALs can vary because of fluctuations of the ionizing continuum or changes in partial coverage while NALs can vary primarily because of changes in partial coverage.