Emergence and Variability of Broad Absorption Line Quasar Outflows
  We isolate a set of quasars that exhibit emergent C iv broad absorption lines
(BALs) in their spectra by comparing spectra in the SDSS Data Release 7 and the
SDSS/BOSS Data Releases 9 and 10. After visually defining a set of emergent
BALs, follow-up observations were obtained with the Gemini Observatory for 105
quasars. We find an emergence rate consistent with the previously reported
disappearance rate of BAL quasars given the relative numbers of non-BAL and BAL
quasars in the SDSS. We find candidate newly emerged BALs are preferentially
drawn from among BALs with smaller balnicity indices, shallower depths, larger
velocities, and smaller widths. Within two rest-frame years (average) after a
BAL has emerged, we find it equally likely to continue increasing in equivalent
width in an observation six months later (average) as it is to start
decreasing. From the time separations between our observations, we conclude the
coherence time-scale of BALs is less than 100 rest-frame days. We observe
coordinated variability among pairs of troughs in the same quasar, likely due
to clouds at different velocities responding to the same changes in ionizing
flux; and the coordination is stronger if the velocity separation between the
two troughs is smaller. We speculate the latter effect may be due to clouds
having on average lower densities at higher velocities due to mass conservation
in an accelerating flow, causing the absorbing gas in those clouds to respond
on different timescales to the same ionizing flux variations.
