Neutron stars can do it too if they want to

Abstract

Neutron stars typically appear as hard X-ray sources. This is because the emission occurs near the neutron-star surface and the lower limit to the characteristic temperature of emission is of order 1 keV. In order to have characteristic temperature of 50 eV and near-Eddington luminosity, the photosphere must be at 1000 neutronstar radii. For sub-Eddington luminosities, a spherically symmetric accretion flow onto a neutron star is optically thin. For near-Eddington luminosities, the accretion flow can be either optically thin or optically thick. It all depends on how the spherically symmetric flow starts at large distances from the neutron star surface. If it starts subsonically, the flow remains subsonic, the velocity decreases with decreasing radius, the optical depth of the flow is huge and the photosphere is pushed out. Thus, a neutron star could, under special conditions, appear as a supersoft source. Such special conditions may occur in transient X-ray sources. Indeed, the source RX J0059.2-7138 is a transient X-ray pulsar in the Small Magellanic Cloud with 90% of its luminosity emitted as a supersoft X-ray spectrum.