Astronomy Colloquium

  Actively accreting supermassive black holes are found, nearly universally, to create luminous X-ray emission, and this point underlies the utility of X-ray surveys for finding active galactic nuclei throughout the Universe. However, there are apparent X-ray weak exceptions to this rule that are now providing novel insights, including weak-line quasars (WLQs) and especially analogs of the extreme WLQ, PHL 1811. We have been systematically studying such X-ray weak quasars with Chandra and near-infrared spectroscopy, and I will report results on their remarkable properties and describe implications for models of the accretion disk/corona, emission-line formation, and quasar winds. We havefound evidence that many of these quasars may have geometrically thick inner accretion disks, likely due to high accretion rates, that shield the high-ionization broad line region from the ionizing continuum. Such shielding may, more generally, play a significant role in shaping the broad distributions of quasar C IV emission-line equivalent widths and blueshifts. Furthermore, I will report NuSTAR observations indicating that a significant fraction of BAL quasars are intrinsically X-ray weak, thereby promoting strong wind driving. I will end by discussing some promising ongoing studies that are extending these ideas.