A Super-Eddington Compton-Thick Wind in GRO J1655-40?

TL;DR

This paper proposes that the hypersoft state of GRO J1655-40 during its 2005 outburst is caused by a super-Eddington Compton-thick wind, which explains the observed spectral and variability features.

Contribution

It introduces a model where a super-Eddington wind accounts for the unique spectral and variability characteristics of GRO J1655-40's hypersoft state.

Findings

Detection of a dense, extended outflow from the accretion disk.

The optical/infrared blackbody corresponds to the wind's photosphere.

X-ray emission is dominated by Compton scattering in the wind.

Abstract

During its 2005 outburst, GRO J1655-40 was observed at high spectral resolution with the Chandra HETGS, revealing a spectrum rich with blueshifted absorption lines indicative of an accretion disk wind -- apparently too hot, too dense, and too close to the black hole to be driven by radiation pressure or thermal pressure (Miller et al.). But this exotic wind represents just one piece of the puzzle in this outburst, as its presence coincides with an extremely soft and curved X-ray continuum spectrum, remarkable X-ray variability (Uttley & Klein-Wolt), and a bright, unexpected optical/infrared blackbody component that varies on the orbital period. Focusing on the X-ray continuum and the optical/infrared/UV spectral energy distribution, we argue that the unusual features of this "hypersoft state" are natural consequences of a super-Eddington Compton-thick wind from the disk: the optical/infrared blackbody represents the cool photosphere of a dense, extended outflow, while the X-ray emission is explained as Compton scattering by the relatively cool, optically thick wind. This wind obscures the intrinsic luminosity of the inner disk, which we suggest may have been at or above the Eddington limit.