I want to better understand how matter accretes onto black holes through accretion disks, the physical mechanisms that produce radiation at different wavebands, and how much energy accreting black holes can deposit into their environments (through jets and winds). I am especially interested in how the above properties change and evolve at different accretion rates.
I believe that the best way to gain a complete picture of black hole accretion is by piecing together clues from different types of black holes, and by looking at the entire electromagnetic spectrum. Each type of black hole offers a unique view of the same basic phenomenology, often highlighting specific physical mechanism(s) that are difficult to isolate in other types of systems. To take advantage of these different perspectives, my research covers a range of subfields of black hole physics, including black hole X-ray binaries, ultraluminous X-ray sources, relativistic jets, quasar emission line regions, and blazars. I also employ observational techniques across the entire electromagnetic spectrum, from the radio through X-ray.
Most of my work on stellar mass black holes has focused on the lowest accretion rates, when black hole X-ray binaries (BHXBs) are in the ‘quiescent’ spectral state (Eddington ratios <10^-5). BHXBs are known to have “softer” (i.e., steeper) X-ray spectra in quiescence compared to when in the more luminous “hard” spectral state (Eddington ratios between 10^-4 -- 10^-2), and much of my work centers around trying to understand the reason, while also exploring the connection between accretion flows and their relativistic outflows.
I have approached this problem by monitoring black hole X-ray binaries as they re-enter quiescence after an outburst, through broadband modeling of quiescent spectral energy distributions, and through long- and short-term variability studies of quiescent black holes.
I got my start working on supermassive black holes and their jets, where my PhD thesis involved mining the Sloan Digital Sky Survey for BL Lac objects (weakly accreting blazars, where a jet is pointing nearly towarrd the Earth). After some serendipitous discoveries, my PhD work led me toward studying a rare phenomenon called Weak Line Quasars, which are quasars that appear to be missing some of their broad emisision lines (which is usually one of the hallmark signatures for quasars). Most likely this is because their broad line regions are in an unusual photoionized state. I have been involved in various work on their multiwavelength properties, and I have led programs to extend their spectral coverage into the near-infrared (with the Very Large Telescope) and into the ultraviolet (Hubble Space Telescope).
I am also working on the smallest known SMBHs (~10^4-10^6 Solar masses) in the nuclei of low-mass galaxies. I have worked on their X-ray properties, as well as multiwavelength searches to discover new candidates. Lately, I am developing programs to clarify the nature of their radio emission across a range of accretion rates. My interest in these “small” SMBHs is to: (1) to observationally constrain how accretion and jet properties may depend on black hole mass; (2) to understand their radiative signatures to guide new surveys for discovering new candidates; (3) to eventually better understand how efficiently black holes may have grown to supermassive sizes in the early Universe through accretion.
Finally, one of my ultimate goals is to synthesize our knowledge on stellar and supermassive black holes to gain a more complete picture of black hole accretion and jets, at least to the level that the physics is scale invariant. One example of this is work I did on the fundamental plane of black hole activity, which is a non-linear relation between radio luminosity, X-ray luminosity, and black hole mass for “hard state” black hole X-ray binaries and their supermassive analogs. The fundamental plane is often appealed to as a means to estimate black hole masses for candidate black holes with radio and X-ray luminosity measurements, and it is often used to discover low-mass black holes in small galaxies. I am currently working on projects designed to determine how to more efficiently use the fundamental plane to identify new black holes via their multiwavelength emisison.