Connecting the X-ray properties of weak-line and typical quasars:
  testing for a geometrically thick accretion disk
  We present X-ray and multiwavelength analyses of 32 weak emission-line
quasars (WLQs) selected in a consistent and unbiased manner. New $Chandra$
3.1-4.8 ks observations were obtained for 14 of these WLQs with C IV rest-frame
equivalent widths (REWs) of 5-15 \r{A}, and these serve as an X-ray
observational "bridge" between previously studied WLQs with C IV REW $\lesssim$
5 \r{A} and more-typical quasars with C IV REW $\approx$ 15-100 \r{A}. We have
identified and quantified a strong dependence of the fraction of X-ray weak
quasars upon C IV REW; this fraction declines by a factor of $\approx 13$ (from
$\approx 44$% to $\approx 3$%) for C IV REW ranging from 4-50 \r{A}, and the
rate of decline appears particularly strong in the 10-20 \r{A} range. The
dependence broadly supports the proposed "shielding" model for WLQs, in which a
geometrically and optically thick inner accretion disk, expected for a quasar
accreting at a high Eddington ratio, both prevents ionizing EUV/X-ray photons
from reaching the high-ionization broad emission-line region and also sometimes
blocks the line-of-sight to the central X-ray emitting region. This model is
also supported by the hard average spectral shape of X-ray weak WLQs (with a
power-law effective photon index of $\Gamma_{\rm eff}=1.19^{+0.56}_{-0.45}$).
Additionally, we have examined UV continuum/emission-line properties that might
trace X-ray weakness among WLQs, confirming that red UV continuum color is the
most-effective tracer.
