An optically-thick disk wind in GRO J1655-40?

TL;DR

This study reanalyzed the wind in GRO J1655-40, suggesting it is optically thick and likely driven by magnetic forces, with implications for understanding wind-driving mechanisms in black hole binaries.

Contribution

It introduces a new irradiated disk model to interpret multi-wavelength data, revealing the wind's optical thickness and high intrinsic luminosity, challenging previous assumptions.

Findings

The wind is likely optically thick and magnetically driven.

The intrinsic luminosity at the Chandra epoch exceeds 0.7 L_Edd.

Scattering effects significantly influence observed X-ray luminosity.

Abstract

We revisited the unusual wind in GRO J1655-40 detected with Chandra in 2005 April, using long-term RXTE X-ray data and simultaneous optical/near-infrared photometric data. This wind is the most convincing case for magnetic driving in black hole binaries, as it has an inferred launch radius which is a factor of 10 smaller than the thermal wind prediction. However, the optical and near-infrared fluxes monotonically increase around the Chandra observation, whereas the X-ray flux monotonically decreases from 10 days beforehand. Yet the optical and near-infrared fluxes are from the outer, irradiated disk, so for them to increase implies that the X-rays likewise increased. We applied a new irradiated disk model to the multi-wavelength spectral energy distributions (SEDs). Fitting the optical and near-infrared fluxes, we estimated the intrinsic luminosity at the Chandra epoch was > ~0.7 L_Edd, which is more than one order of magnitude larger than the observed X-ray luminosity. These results could be explained if a Compton-thick, almost completely ionized gas was present in the wind and strong scattering reduced the apparent X-ray luminosity. The effects of scattering in the wind should then be taken into account for discussion of the wind-driving mechanism. Radiation pressure and Compton heating may also contribute to powering the wind at this high luminosity.