Multiwavelength Signatures of Super-Eddington Accretion in Active Galactic Nuclei
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Authors
Paul, Jeremiah D.
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
accretion , active galactic nuclei , black holes , multiwavelength astronomy , quasars , slim disks
Alternative Title
Abstract
Quasars are present at high redshift, suggesting that some supermassive black holes grew quickly through accretion and/or mergers soon after the Big Bang. Growth through accretion likely required episodes exceeding the Eddington limit. To improve our understanding of how efficiently seed black holes can be fed, we need better constraints on the observational characteristics of near/super-Eddington accretion in active galactic nuclei (AGNs) -- particularly changes to the structure and radiative output of the central engine. Accretion disks in the standard AGN model are geometrically thin and radiatively efficient at low-to-moderate accretion rates, but above near-Eddington rates the disk may inflate at small radii, becoming a geometrically thick and radiatively inefficient "slim" disk. This change facilitates a transition to super-Eddington accretion, and it undoubtedly impacts the appearance and radiative output of the AGN central engine and its surrounding regions, necessitating multiwavelength studies. I performed tests using new and archival observations across radio, optical, ultraviolet, and X-ray bands, from which I find support for the slim-disk accretion state in samples of both supermassive and low-mass AGNs. First, I used optical, ultraviolet, and X-ray observations to examine a sample of quasars with unusually weak broad emission lines, and I found that their emission-line weakness may result from a slim disk shielding the broad emission-line region gas from ionizing radiation. Next, I used radio and X-ray observations to examine a sample of low-mass AGNs that display a puzzling trend toward unusual X-ray weakness, which I found may result from a slim disk shielding their X-ray emission from our line of sight. Finally, I examined the origin of radio emission in a sample of radio-quiet AGNs in the context of how a slim disk is likely to alter the optical, X-ray, and outflow characteristics of the central engine, and I found that they may be dominated by radio emission from a compact corona with the addition of uncollimated outflows at higher accretion rates. I conclude with a brief discussion of future observational prospects to improve our understanding of the interaction between AGN emission mechanisms and the efficiency of black hole growth through accretion.