Simulating state transitions in black hole accretion flows
Olek Sadowski, MIT
Galactic BH X-ray binaries often undergo state transitions during which their luminosities and spectrum significantly change. Accretion in distinct modes is also observed in AGN. In this talk I will review most recent progress in simulating transitions from optically thin to optically thick black hole accretion flows occurring during the onset of the outbursts, when the accretion rate on the BH increases.
Rates of Tidal Disruption Events: Both Too High and Too Low
Nick Stone, Columbia
Abstract: When an unlucky star passes too close to a supermassive black hole, it is torn apart in a tidal disruption event (TDE). Half the stellar debris remains bound to the black hole, and as it returns to pericenter it begins to accrete at super-Eddington rates, powering a multiwavelength flare comparable in luminosity to the brightest supernovae. As our sample size of observed TDEs has increased, two opposing event rate dilemmas have emerged. In typical galaxies, the observationally inferred rate of TDEs is 1-2 orders of magnitude below theoretically predicted rates that are computed using the Fokker-Planck approximation for collisional stellar dynamics. However, TDEs are overproduced in E+A galaxies, a rare post-starburst galaxy subtype that makes up ~0.1% of local universe galaxies yet hosts ~50% of all TDEs. I will discuss my ongoing observational and theoretical work to resolve these parallel rate discrepancies. Preliminary evidence suggests that the underproduction of TDEs in standard galaxies may be linked to a bimodality in the TDE luminosity function, while the overproduction of TDEs in E+As may be due to central stellar overdensities produced in the starburst that made the E+A.