CAN A SPHERICALLY ACCRETING BLACK-HOLE RADIATE VERY NEAR THE EDDINGTON LIMIT

I discuss some aspects of radiative transfer through a supercritical, spherically symmetric accretion flow on to a black hole, and conclude that the diffusive luminosity at infinity cannot exceed 0.6 LE. All of the emergent radiation is produced by the gas during infall, from energy originally supplied by the gravitational field. When the gas has fallen far enough for its emissivity to approach LE, it is on the verge of crossing the 'trapping radius' for radiation, and much of the radiation is dragged into the hole. On the basis of this result, I argue that radiation pressure from a spherically accreting black hole cannot prevent the accretion rate from attaining arbitrarily high values, even if 'photon bubbles' transport much of the radiation. In a concluding section, I discuss the possible astrophysical relevance of supercritically accreting black holes, and describe how one might appear to an observer. © Royal Astronomical Society.