Sustained super-Eddington accretion around neutron stars & black holes

Abstract

Recently, it was shown that the formation of a photon-trapping surface might not be sufficient to ensure unimpeded super-Eddington (SE) accretion. In light of this finding, here we derive a condition such that sustained and unimpeded SE accretion could be achieved in optically thick slim accretion discs surrounding neutron stars (NSs) and black holes (BHs). For this, we calculate a semi-analytic approximation of the self-similar global radial velocity expression for an advection-dominated flow. Neglecting the influence of relativistic jets on the accretion flow, we find that for Eddington fraction m· ≳ 1.5(∈/0.1)3/5 (∈ being the accretion efficiency) sustained SE accretion might be possible in slim discs around BHs irrespective of their spin. The same condition holds for NSs when ∈ ≳ 0.03. The presence of a surface magnetic field might truncate the disc at the magnetosphere of the NS, resulting in lower efficiencies and consequently changing the condition to m· > 0.013∈−19/31. Our approach suggests that sustained SE accretion might almost always be possible around NSs and BHs hosting accretion discs.