Black holes: accretion + hysteresis

My research, carried out under the aegis of Prof. David Smith here at U.C. Santa Cruz, uses a blend of theory and observations to explain the observed hysteresis phenomenon in stellar mass black hole binaries.

Throughout my career, I plan to continue doing work that requires a roughly 40/60 split between theory and observations.

Building a model of black hole binary (BHB) accretion that fully explains the hysteresis phenomenon will be the primary focus of my thesis work here at U.C. Santa Cruz. In a hysteretic BHB, the hardness of the source's x-ray spectrum depends, in a complex and poorly-understood way, on the recent history of that source's luminosity (see figure). The specific structure of the accretion flow may lie at the root of this phenomenon.

An illustration of hysteresis, as it typically appears in stellar-mass black hole binary systems. Transient sources move through states A, B, and C in order. Persistant systems, however, make partial transitions between states A and B, or between states B and C. My work tries to explain the physics driving these partial state transitions.

Most BHB accretion flows contain an accretion disk, a corona of high-energy electrons, and, in some cases, jets. Understanding how each of these flow components, with its characteristic spectrum and luminosity, depends on and evolves with the others provides a promising means to explain hysteresis. Building an accurate model of how these accretion flow components evolve and interact requires reliable observational constraints on their luminosities.

Multiwavelength observations are key to constraining how much energy each accretion flow component carries away. X-ray observations, which I currently carry out with the Rossi X-ray Timing Explorer (RXTE) satellite, give good estimates for the accretion disk and coronal luminosities. To see how much energy jets carry away from the system, however, one must turn to radio and optical observations.

Black hole hysteresis is only the first of many areas of research related to accretion flows that I plan to tackle during the course of my career. In particular, I am also interested in using models and observations of accretion onto stellar-mass black holes to put clearer limits on AGN feedback and evolution.