Monte Carlo Neutrino Transport in Post-merger Disks

Sherwood Richers, California Institute of Technology

We present Sedonu, a new open-source, steady-state, special relativistic Monte Carlo (MC) neutrino transport code, available at bitbucket.org/srichers/sedonu. We solve for the energy- and angle-dependent neutrino distribution function on fluid backgrounds of zero to three spatial dimensions, calculate the rates of change of fluid temperature and electron fraction, and solve for the equilibrium fluid temperature and electron fraction. We apply this method to fluid in snapshots from two-dimensional simulations of accretion disks left behind by binary neutron star mergers, varying the input physics and comparing to the results obtained with a leakage scheme for both the cases of a central black hole and a central hypermassive neutron star. Neutrinos are guided away from the densest regions of the disk and escape preferentially around 45 degrees from the equatorial plane. The neutrino rates a few scale heights above the disk midplane near the innermost stable circular orbit are increased by MC transport at early times, perhaps leading to a stronger neutrino-driven wind. Neutrino cooling in the dense midplane of the disk is larger when using MC transport, leading to a globally larger cooling rate by a factor of a few and a larger leptonization rate by an order of magnitude. We calculate neutrino-pair annihilation rates and find that in the presence of a hypermassive neutron star, the neutrino-pair annihilation deposits 2.8e46 erg when a central BH is present or 1.9e48 erg when a HMNS is present within 45 degrees of the poles, neither of which is likely to be sufficient to drive a GRB jet.

Abstract Author(s): Sherwood Richers, Dan Kasen, Evan O'Connor, Rodrigo Fernandez, Christian Ott