Global neutrino heating in hyperaccretion flows

Abstract

The neutrino-dominated accretion flow (NDAF) with accretion rates Ṁ = 0.01-10M⊙ s-1 is a plausible candidate for the central engine of gamma-ray bursts (GRBs). This hyperaccretion disc is optically thin to neutrinos in the radial direction; therefore, the neutrinos produced at one radius can travel for a long distance in the disc. Those neutrinos can thus be absorbed with certain probability by the disc matter at the other radius and heat the disc there. The effect of this 'global neutrino heating' has been ignored in previous works and is the focus of this paper. We find that around the 'ignition' radius rign, the global neutrino heating rate could be comparable to or even larger than the local viscous heating rate and thus it must be an important process. Two possible consequences are in order if the 'global neutrino heating' is taken into account: (i) the temperature of the disc is slightly raised and the 'ignition' radius rign slightlyshifts to a larger radius, both lead to the increase of the total neutrino flux; (ii) what is more interesting is that the temperature of the ADAF just beyond rign may be raised above the virial temperature; thus, the accretion will be suppressed. In this case, the activity of the black hole is expected to oscillate between active and inactive phases. The time-scale of the active phases is estimated to be ~1 s. If the time-scale of the inactive phase is comparable to or less than this value, this intermittent activity may explain the slow variability component of the GRBs. Self-consistent global calculations of NDAFs with the 'global neutrino heating' included are required in the future to more precisely evaluate this effect. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.