A strange and violent fate awaits a white dwarf star that wanders too close to a moderately massive black hole. As it passes the black hole, the white dwarf becomes strongly compressed and heated, triggering an explosion. Most of the stellar mass is ejected into space, while the rest falls toward the black hole. While the ejected matter expands rapidly, the infalling matter builds a violent, thick accretion disk around the black hole.

I am interested in the dynamics of plasma in extreme astrophysical environments. My current goal is to combine high quality numerical simulations with analytical techniques in order to address basic questions in high energy astrophysics. Since my arrival at Santa Cruz, I have been involved in various theoretical aspects of gamma-ray burst, supernova, neutron stars, stellar evolution and dynamics, cosmology, and pulsar physics.  Some of my current research interests include:

Few would dispute the statement that the photons which bring us ample information about the nature of GRBs, supernova remnants, magnetar flares, and pulsars are the result of particle acceleration in relativistic shocks. Since charged particles radiate only when accelerated, one must attempt to deduce from the spectrum how the particles are being accelerated, why they are being accelerated, and to identify the macroscopic source driving the micro physical acceleration process.

Recent years have witnessed dramatic progress in our understanding of how turbulence arises and transports angular momentum in differentially rotating systems. The key conceptual point is that the combination of a sub thermal magnetic field and outwardly decreasing differential rotation rapidly generates magneto hydrodynamical turbulence via the magnetorotational instability. With the aid of supercomputers, it is now possible to study accretion disk turbulence at a level comparable to that of stellar convection.

encounters in dense stellar environments

particle acceleration and cooling in relativistic shocks

dynamics, heating and cooling of accretion disks

GRB sources involve energies that can exceed the mass equivalent of 1/100 of a sun. Compared with the size of the sun, the seat of this activity is extraordinarily compact, as indicated by rapid variability of the radiation flux on  millisecond timescales. It is unlikely that mass can be converted into energy with better than a few per cent efficiency; therefore, the more powerful GRB sources must process upwards a 1/10 of a solar mass through a region which is not much larger than the size of a neutron star or a stellar mass black hole. No other entity can convert mass to energy with such a high efficiency, or within such a small volume.

triggering gamma-ray bursts