Variability in Quasar Broad Absorption Line Outflows I. Trends in the
  Short-Term versus Long-Term Data
  Broad absorption lines (BALs) in quasar spectra identify high velocity
outflows that likely exist in all quasars and could play a major role in
feedback to galaxy evolution. The variability of BALs can help us understand
the structure, evolution, and basic physical properties of the outflows. Here
we report on our first results from an ongoing BAL monitoring campaign of a
sample of 24 luminous quasars at redshifts 1.2<z<2.9, focusing on C IV 1549 BAL
variability in two different time intervals: 4 to 9 months (short-term) and 3.8
to 7.7 years (long-term) in the quasar rest-frame. We find that 39% (7/18) of
the quasars varied in the short-term, whereas 65% (15/23) varied in the
long-term, with a larger typical change in strength in the long-term data. The
variability occurs typically in only portions of the BAL troughs. The
components at higher outflow velocities are more likely to vary than those at
lower velocities, and weaker BALs are more likely to vary than stronger BALs.
The fractional change in BAL strength correlates inversely with the strength of
the BAL feature, but does not correlate with the outflow velocity. Both the
short-term and long-term data indicate the same trends. The observed behavior
is most readily understood as a result of the movement of clouds across the
continuum source. If the crossing speeds do not exceed the local Keplerian
velocity, then the observed short-term variations imply that the absorbers are
<6 pc from the central quasar.
