Exceptional X-ray Weak Quasars: Implications for Accretion Flows

Actively accreting supermassive black holes (SMBHs) are found, nearly universally, to create luminous X-ray emission, and this point underlies the utility of X-ray surveys for finding growing SMBHs throughout the Universe. However, there are exceptions to this rule that provide novel insights, including PHL 1811 analogs and some weak-line quasars. We have been systematically studying such X-ray weak quasars with the Chandra X-ray Observatory, aiming (1) to define their optical-to-X-ray spectral energy distributions, (2) to measure their basic X-ray spectral properties, and (3) to establish the optical/UV emission-line and continuum properties that most directly trace X-ray weakness. Many of these type 1 quasars show unusually hard X-ray spectra, suggesting that small-scale absorption/reflection has a primary role in causing their X-ray weakness and distinctive emission-line properties. Physical considerations indicate that this small-scale absorber/reflector may be the geometrically thick inner accretion disk expected to form if PHL 1811 analogs and weak-line quasars have unusually high SMBH accretion rates.