# Evidence for Optically Thick, Eddington-limited Winds Driven by Supercritical Accretion

## TL;DR

This paper presents evidence that supercritical accretion onto compact objects drives optically thick, Eddington-limited winds, explaining a class of luminous, soft X-ray sources with spectra consistent with wind photospheres.

## Contribution

It provides observational evidence linking supercritical accretion-driven winds to luminous soft X-ray sources, supporting the theoretical model of optically thick, Eddington-limited winds.

## Key findings

- Most sources are associated with late-type galaxies.
- Sources are too hot or luminous to be white dwarf nuclear burning.
- Blackbody luminosities suggest both neutron stars and stellar-mass black holes.

## Abstract

Supercritical accretion onto compact objects powers a massive wind that is optically thick and Eddington-limited. If most of the hard X-rays from the central disk are obscured by the wind, the source will display a blackbody-like spectrum with a luminosity scaled with the mass of the compact object. From the Chandra archive of nearby galaxies, we selected a sample of luminous and very soft sources and excluded contaminations from foreground objects and supernova remnants. They are found to be preferentially associated with late-type galaxies. The majority of sources in our sample are either too hot or too luminous to be explained by nuclear burning on the surface of white dwarfs, and are argued to be powered by accretion. The most likely explanation is that they are due to emission from the photosphere of a wind driven by supercritical accretion onto compact objects. Their blackbody luminosity ranges from ~10^37 to nearly 10^40 erg/s, indicative of the presence of both neutron stars and stellar-mass black holes. The blackbody luminosity also shows a possible bimodal distribution, albeit at low significance, peaked around the Eddington limit for neutron stars and stellar-mass black holes, respectively. If this can be confirmed, it will be smoking gun evidence that supercritical accretion powers thick winds. Based on a wind model, the inferred mass accretion rate of these objects is around a few hundred times the Eddington rate, suggesting that they may be intermediate between the canonical ultraluminous X-ray sources and SS 433 in terms of the accretion rate.