Probing the broad-line regions in active galactic nuclei using time variability of associated absorption lines

It is known that most of the clouds that produce associated absorption in t he spectra of AGNs and quasars do not completely cover the background sourc e, i.e., the continuum plus the broad-line region (BLR). We note that the c overing factor derived for the absorption is the fraction of photons occult ed by the absorbing clouds and is not necessarily the same as the fractiona l area covered. We show that the variability in absorption lines can be pro duced by changes in the covering factor caused by variation in the continuu m and the finite light travel time across the BLR. We discuss how such vari ability can be distinguished from variability caused by other effects and h ow one can use the variability in the covering factor to probe the BLR.