Seeking Multiwavelength Signs of Near-Eddington Accretion in Active Galactic Nuclei

The presence of quasars at z > 6 suggests that soon after the Big Bang, some supermassive black holes (M_BH ≳ 10⁶ M_Sun) grew quickly through accretion and/or mergers. Growth through accretion likely required episodes of accretion near/exceeding the Eddington limit. However, to understand how efficiently seed black holes could grow through rapid accretion, better constraints are required on observational characteristics near the Eddington limit, especially the impact on the structure of the central engine of the active galactic nucleus (AGN). Here, we focus on an optically-selected sample of rapidly-accreting, “massive” black holes (M_BH ≈ 10⁵–10⁶ M_Sun) in AGNs identified by Greene & Ho (2007) from the Sloan Digital Sky Survey. Contrary to typical AGNs, which show an anticorrelation between ultraviolet (thermal disk) emission and hard X-ray (coronal) emission, the Greene & Ho catalog displays a perplexing X-ray-weak tail where the coronal radiative output can be up to 100× weaker than expected. We present new and archival radio observations from the Very Large Array in combination with archival X-ray observations from Chandra for a sample of this catalog. Through multiwavelength analysis, we demonstrate that while nearly all Greene & Ho AGNs are radio-quiet, the X-ray-weak objects have larger radio/X-ray luminosity ratios than their X-ray-normal counterparts. This trend is not observed with typical radio-quiet AGNs, where the radio/X-ray luminosity ratio is usually ~constant, independent of X-ray properties. We argue that X-ray weakness in our sample may result from slim-disk accretion, where a puffed-up inner disk and related outflows cause orientation-dependent X-ray (but not radio) shielding, and we relate this observation to existing work on weak emission-line quasars (AGNs with remarkably weak broad-line emission and a significant X-ray-weak fraction). We briefly discuss in-progress work extending these results to diagnosis of the dominant radio emission mechanism(s). We conclude by suggesting new observational signatures for finding rapidly-accreting AGNs. Such samples will improve our understanding of near-Eddington accretion and the level to which black holes can grow through accretion.