On the origins of C IV absorption profile diversity in broad absorption
  line quasars
  There is a large diversity in the C IV broad absorption line (BAL) profile
among BAL quasars (BALQs). We quantify this diversity by exploring the
distribution of the C IV BAL properties, FWHM, maximum depth of absorption and
its velocity shift ($v_{\rm md}$), using the SDSS DR7 quasar catalogue. We find
the following: (i) Although the median C IV BAL profile in the quasar
rest-frame becomes broader and shallower as the UV continuum slope
($\alpha_{\rm UV}$ at 1700-3000 A) gets bluer, the median individual profile in
the absorber rest-frame remains identical, and is narrow (FWHM = 3500 km/s) and
deep. Only 4 per cent of BALs have FWHM > 10,000 km/s. (ii) As the He II
emission equivalent-width (EW) decreases, the distributions of FWHM and $v_{\rm
md}$ extend to larger values, and the median maximum depth increases. These
trends are consistent with theoretical models in which softer ionizing continua
reduce overionization, and allow radiative acceleration of faster BAL outflows.
(iii) As $\alpha_{\rm UV}$ becomes bluer, the distribution of $v_{\rm md}$
extends to larger values. This trend may imply faster outflows at higher
latitudes above the accretion disc plane. (iv) For non-BALQs, the C IV emission
line decreases with decreasing He II EW, and becomes more asymmetric and
blueshifted. This suggests an increasing relative contribution of emission from
the BAL outflow to the C IV emission line as the ionizing spectral energy
distribution (SED) gets softer, which is consistent with the increasing
fraction of BALQs as the ionizing SED gets softer.
