What Determines Unique Spectra of Super-Eddington Accretors?: Origin of Optically Thick and Low Temperature Coronae in Super-Eddington Accretion Flows

TL;DR

This paper investigates the origin of cool, optically thick coronae in super-Eddington accretion flows, proposing that radiation pressure-driven winds heated by magnetic reconnection can explain their properties, unlike in sub-Eddington flows.

Contribution

It introduces a model where radiation pressure-driven winds form optically thick coronae in super-Eddington flows, reproducing observed coronal properties and contrasting with sub-Eddington cases.

Findings

Radiation pressure-driven winds can form optically thick coronae in super-Eddington flows.

The model reproduces observed coronal optical depth and temperature.

Low temperature, optically thick coronae are signatures of super-Eddington accretion.

Abstract

Existence of relatively cool ($k_B T \lesssim 10~{\rm keV}$) and optically thick ($\tau \gtrsim 3$) coronae are inferred above super-Eddington accretion flow such as ultraluminous X-ray sources (ULXs), GRS 1915+105, and narrow-line Seyfert 1 galaxies (NLS1), which contrasts the cases in sub-Eddington accretion flows, which are associated with coronae with $k_B T \sim 100~{\rm keV}$ and $\tau \sim 1$. 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 emerged 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 support that the low temperature, optically thick Comptonization should be a key signature of super-Eddington accretion flow.