Appearance vs Disappearance of broad absorption line troughs in quasars
  We present a new set of 84 Broad absorption line (BAL) quasars ( 1.7 $<$ \zem
$<$ 4.4) exhibiting an appearance of \civ BAL troughs over 0.3$-$4.8 rest-frame
years by comparing the Sloan Digital Sky Survey Data Release (SDSSDR)-7,
SDSSDR-12, and SDSSDR-14 quasar catalogs. We contrast the nature of BAL
variability in this appearing BAL quasar sample with a disappearing BAL quasar
sample studied in literature by comparing the quasar's intrinsic, BAL trough,
and continuum parameters between the two samples. We find that appearing BAL
quasars have relatively higher redshift and smaller probed timescales as
compared to the disappearing BAL quasars. To mitigate the effect of any
redshift bias, we created control samples of appearing and disappearing BAL
quasars that have similar redshift distribution. We find that the appearing BAL
quasars are relatively brighter and have shallower and wider BAL troughs
compared to the disappearing BAL sample. The distribution of quasar continuum
variability parameters between the two samples is clearly separated, with the
appearance of the BAL troughs being accompanied by the dimming of the continuum
and vice versa. Spectral index variations in the two samples also point to the
anti-correlation between the BAL trough and continuum variations consistent
with the "bluer when brighter" trend in quasars. We show that the intrinsic
dust model is less likely to be a favorable scenario in explaining BAL
appearance/disappearance. Our analysis suggests that the extreme variations of
BAL troughs like BAL appearance/disappearance are mainly driven by changes in
the ionization conditions of the absorbing gas.
