X-ray and Multiwavelength Insights into the Nature of Weak Emission-Line
  Quasars at Low Redshift
  (Abridged) We report on the X-ray and multiwavelength properties of 11
radio-quiet quasars with weak or no emission lines identified by the Sloan
Digital Sky Survey (SDSS) with redshift z=0.4-2.5. The distribution of relative
X-ray brightness for our low-redshift weak-line quasar (WLQ) candidates is
significantly different from that of typical radio-quiet quasars, having an
excess of X-ray weak sources, but it is consistent with that of high-redshift
WLQs. The X-ray weak sources generally show similar UV emission-line properties
to those of the X-ray weak quasar PHL 1811; they may belong to the notable
class of PHL 1811 analogs. The average X-ray spectrum of these sources is
somewhat harder than that of typical radio-quiet quasars. Several other
low-redshift WLQ candidates have normal ratios of X-ray-to-optical/UV flux, and
their average X-ray spectral properties are also similar to those of typical
radio-quiet quasars. The X-ray weak and X-ray normal WLQ candidates may belong
to the same subset of quasars having high-ionization "shielding gas" covering
most of the wind-dominated broad emission-line region, but be viewed at
different inclinations. The mid-infrared-to-X-ray spectral energy distributions
(SEDs) of these sources are generally consistent with those of typical SDSS
quasars, showing that they are not likely to be BL Lac objects with
relativistically boosted continua and diluted emission lines. However, one
source in our X-ray observed sample is remarkably strong in X-rays, indicating
that a small fraction of low-redshift WLQ candidates may actually be BL Lacs
residing in the radio-faint tail of the BL Lac population. We also investigate
universal selection criteria for WLQs over a wide range of redshift, finding
that it is not possible to select WLQ candidates in a fully consistent way
using different prominent emission lines as a function of redshift.
