2019ApJ...883...30L

We discover the significant (significance level of >99%) correlations between the fractional variation of the ionizing continuum and that of the C IV and/or Si IV broad absorption lines (BALs) in each of 21 BAL quasars that have at least five-epoch observations from the Sloan Digital Sky Survey-I/II/III. This result reveals that the fluctuation of the ionizing continuum is the driver of most of these BAL variations. Among them, 17 show negative correlations and the other 4 positive correlations, which agrees with the prediction of photoionization models that absorption line variability response to ionization changes is not monotonic. Eight quasars out of 21 examples have been observed at least 30 times on rest-frame timescales as short as a few days, which reveals that changes in the incident ionizing continuum can cause BAL variability even in such a short period of time. In addition, we find that most of the 21 quasars show larger variation amplitude in Si IV than C IV, which reveals the ubiquity of saturation in these BALs (at least for C IV BALs).