Variability of Narrow, Associated Absorption Lines in Moderate‐ and Low‐Redshift Quasars

We present the results of a search for variability in the equivalent widths(EWs) of narrow, associated (Delta v < 5,000 km/s) absorption lines found inthe UV spectra of z < 1.5 quasars. The goal of this search was to usevariability as a means of identifying absorption lines arising in gas that isintrinsic to the quasar central engine. We have compared archival HST/FOSspectra of quasars with recent spectra obtained as part of our own snapshotsurvey of the same objects with STIS. The intervals between observations are4-10 years. We primarily focused on the C IV absorption lines, although we alsostudied other lines available in the same spectra (e.g., Ly-alpha, N V, O VI).Our main result is that 4 out of 15 quasars, or 4 out of 19 associatedabsorption systems, contained variable narrow absorption lines, which areindicative of intrinsic absorption. We conclude that a minimum of 21% of theassociated absorption-line systems are variable. Because not all systems willhave necessarily varied, this is a lower limit on this fraction and it isconsistent with previous estimates based on variability, partial coverageanalysis, or statistical arguments. If we interpret the upper limits on thevariability time scale as upper limits on the recombination time of theabsorber, we constrain the density of the absorber to be n_e > 3000 cm^{-3} andits distance from the ionizing source to be R < 100pc. Moreover, we are nowable to pick out specific intrinsic absorption-line systems to be followed upwith high-dispersion spectroscopy in order to constrain the geometry, location,and physical conditions of the absorber. We briefly illustrate how followupstudies can yield such constraints by means of a simulation.Comment: 42 pages, including 2 tables and 8 figures. To appear in ApJ, vol. 613, 20 Sep. 200