What determines the unique spectra of super-Eddington accretors? Origin of optically thick and low-temperature coronae in super-Eddington accretion flows

Abstract The existence of relatively cool (kBT ≲ 10 keV) and optically thick (τ ≳ 3) coronae are inferred above super-Eddington accretion flow such as ultraluminous X-ray sources, GRS 1915+105, and narrow-line Seyfert 1 galaxies, which contrasts with cases in sub-Eddington accretion flows, which are associated with coronae with kBT ∼ 100 keV and τ ∼ 1. To understand their physical origin, we investigate the emission properties of the corona which is formed by the gas blown off the super-Eddington inner disk by radiation pressure. We assume that the corona is heated by the reconnection of magnetic loops emerging from the underlying disk. We show that this radiation-pressure-driven wind can act as an optically thick corona which upscatters thermal soft photons from the underlying disk, and that with a reasonable parameter set we can theoretically reproduce the coronal optical depth and temperature which are inferred by spectral fittings of observational data. By contrast, the coronal optical depth cannot be so high in sub-Eddington cases, since the coronal material is supplied from the disk via evaporation and there is a maximum limit on the evaporation rate. We suggest that low-temperature, optically thick Comptonization should be a key signature of super-Eddington accretion flow.