Transient Accretion onto Supermassive Black Holes
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masterson-mmasters-phd-physics-2026-thesis.pdf
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49.77 MB
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Author(s)
Masterson, Megan
Advisor(s)
Kara, Erin
Date Issued
May 2026
Publisher
Massachusetts Institute of Technology
Abstract
Supermassive black holes (SMBHs) play a key role in shaping the universe we
see today, yet we still do not understand how these cosmic giants form or grow to be so massive. Time-domain surveys across the electromagnetic spectrum are dramatically influencing our understanding of SMBH accretion from the inner accretion flow, where the majority of the energy is released, to the obscuring circumnuclear environment at parsec scales. In SMBHs that are steadily accreting from their surroundings, time-domain surveys have revealed that the accretion process can be far more variable than we ever thought possible. In this thesis, I show that these extremely variable systems show dramatic changes to the innermost regions of the accretion flow where X-ray emission is produced. In one particular transient, 1ES 1927+654, I discovered just the second high-confidence X-ray quasi-periodic oscillation from an SMBH and the first with significant frequency evolution. This discovery has implications for corona-jet coupling in SMBHs and prospects for next-generation X-ray and gravitational wave synergies. On the other hand, time-domain surveys are also revealing the typically unobservable dormant SMBHs through tidal disruption events (TDEs), which occur when a star is destroyed by the extreme tidal forces of the SMBH. While most TDEs today are discovered as optical transients, my thesis work shows that optical surveys miss a sizable fraction of TDEs obscured by dust. By searching for dust-obscured TDEs in infrared surveys, I uncovered this missing population, showed that these TDEs primarily occur in star-forming galaxies unlike their optical counterparts, and that they occur at a comparable rate to their optical counterparts. Additionally, I showed that follow-up mid-infrared spectroscopy with JWST can be used to probe the circumnuclear environment in these galaxies, thus providing constraints on what triggers SMBH activity. Together, the work presented in this thesis provides a significant step forward in our understanding of the extremes of SMBH activity and new ways to probe the dominant population of dormant SMBHs in the universe.
MIT Department
Massachusetts Institute of Technology. Department of Physics
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