Neutron star accretion in dense environments

Abstract

For a neutron star in a dense environment, the gravitational energy of accreting matter can be lost to neutrinos. One such environment surrounds a newly formed neutron star in a supernova; for the conditions in SN 1987A, ≲ 0.1 M ⊙ may have fallen back onto the central neutron star on a timescale of hours after the explosion. The mass accretion rate subsequently drops more rapidly than t −1, but steady accretion solutions with an extended shocked envelope around the neutron star and neutrino emission close to the surface are possible. Radiation is trapped in the flow until the mass accretion rate drops to 2 × 10 −4 M ⊙ yr −1 at which point radiation can begin to escape from the shocked envelope at an Eddington limit luminosity. Between this neutrino limit and the Eddington limit, 3 × 10 −8 M ⊙ yr −1, there are no steady, spherical solutions for neutron star accretion. SN 1987A should have reached the neutrino limit within a year of the explosion; the current lack of an Eddington luminosity can be attributed to black hole formation or to a clearing of the neutron star envelope. There should be some conditions under which an explosion plus black hole formation occur, because of either accretion of matter brought back to the center by a reverse shock front or accretion of central matter that a low energy explosion is unable to unbind. Accretion above the neutrino limit also occurs for neutron stars in stellar envelopes. In massive stars, the neutron star may be converted to a black hole if the spiral-in is by gas drag.