Broad absorption line disappearance and emergence using multiple-epoch
  spectroscopy from the Sloan Digital Sky Survey
  We investigate broad absorption line (BAL) disappearance and emergence using
a 470 BAL-quasar sample over < 0.10-5.25 rest-frame years with at least three
spectroscopic epochs for each quasar from the Sloan Digital Sky Survey. We
identify 14 disappearing BALs over < 1.73-4.62 rest-frame years and 18 emerging
BALs over < 1.46-3.66 rest-frame years associated with the CIV 1548,1550 and/or
SiIV 1393,1402 doublets, and report on their variability behavior. BAL quasars
in our dataset exhibit disappearing/emerging CIV BALs at a rate of 2.3 and 3.0
per cent, respectively, and the frequency for BAL to non-BAL quasar transitions
is 1.7 per cent. We detect four re-emerging BALs over < 3.88 rest-frame years
on average and three re-disappearing BALs over < 4.15 rest-frame years on
average, the first reported cases of these types. We infer BAL lifetimes along
the line of sight to be nominally < 100-1000 yr using disappearing CIV BALs in
our sample. Interpretations of (re-)emerging and (re-)disappearing BALs reveal
evidence that collectively supports both transverse-motion and
ionization-change scenarios to explain BAL variations. We constrain a nominal
CIV/SiIV BAL outflow location of < 100 pc from the central source and a radial
size of > 1x10^-7 pc (0.02 au) using the ionization-change scenario, and
constrain a nominal outflow location of < 0.5 pc and a transverse size of ~0.01
pc using the transverse-motion scenario. Our findings are consistent with
previous work, and provide evidence in support of BALs tracing compact flow
geometries with small filling factors.
