Studies of Accretion onto Black Holes

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Some of the most luminous objects observed in the universe are associated with matter accreting onto black holes, either while they are forming, or as mature objects. This is important to study because black holes and their formation impact the evolution of galaxies and the production of atomic elements, as well as being a test case for general relativity and physics in extreme conditions. A research group at Harvard University will study multiple aspects of black hole accretion using computer simulation code developed by the group. The research will include investigations of (i) accretion onto a supermassive black hole when a star is torn apart by the black hole, (ii) the growth of supermassive black holes early in the universe, (iii) black hole accretion dominated by light subatomic particles called neutrinos in the collapse of a massive star, and (iv) similar accretion when two compact stars, called neutron stars, collide and merge. This work will form the bulk of the doctoral thesis of a student who is a member of a group that has been traditionally underrepresented in science. During the last decade and a half, general relativistic (GR) magnetohydrodynamic (MHD) codes have been developed and applied widely to black hole systems. The principal investigator of the project has developed the first GRMHD code to include radiation, a program called KORAL, and a radiation post-processor code, called HEROIC, which computes spectra of simulated models. Three research areas will be explored using simulations with KORAL and HEROIC: (i) Super-Eddington accretion onto a supermassive black hole in a tidal disruption event; (ii) the growth of supermassive black holes between redshift 20, when black hole seeds first formed in the universe, and redshift 6, when distant quasars are directly observed; (iii) Super-Eddington models of quasars and fits to observational data. KORAL will be modified to handle neutrino emission, absorption and transfer in neutrino-dominated accretion flows (NDAFs), which are present during core collapse of rapidly rotating massive stars. It will be applied to two problems: (i) NDAFs corresponding to the regimes of radiatively inefficient accretion, thin disk accretion, and super-neutrino-Eddington accretion, with applications to long gamma-ray bursts. (ii) Post-merger NDAFs in double neutron star mergers, with applications to short gamma-ray bursts.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.