SIMBAD references

It is known that most of the clouds that produce associated absorption in the spectra of AGNs and quasars do not completely cover the background source, i.e., the continuum plus the broad-line region (BLR). We note that the covering factor derived for the absorption is the fraction of photons occulted by the absorbing clouds and is not necessarily the same as the fractional area covered. We show that the variability in absorption lines can be produced by changes in the covering factor caused by variation in the continuum and the finite light travel time across the BLR. We discuss how such variability can be distinguished from variability caused by other effects and how one can use the variability in the covering factor to probe the BLR.