Coronae and Winds from Irradiated Disks in X-ray binaries

TL;DR

This paper investigates the formation of dense winds from irradiated accretion disks in X-ray binaries, using simulations to reconcile observed wind densities with theoretical models and exploring their impact on disk stability.

Contribution

It demonstrates that small modifications to heating and cooling rates can significantly increase wind density and mass loss rates, explaining observations and potential state changes.

Findings

Wind density can be increased by an order of magnitude with minor heating/cooling adjustments.

Mass loss rates can surpass accretion rates by up to two orders of magnitude.

High mass loss may destabilize the accretion disk and trigger state changes.

Abstract

X-ray and UV line emission in X-ray binaries can be accounted for by a hot corona. Such a corona forms through irradiation of the outer disk by radiation produced in the inner accretion flow. The same irradiation can produce a strong outflow from the disk at sufficiently large radii. Outflowing gas has been recently detected in several X-ray binaries via blue-shifted absorption lines. However, the causal connection between winds produced by irradiation and the blue-shifted absorption lines is problematic, particularly in the case of GRO J1655-40. Observations of this source imply wind densities about two orders of magnitude higher than theoretically predicted. This discrepancy does not mean that these `thermal disk-winds' cannot explain blue-shifted absorption in other systems, nor that they are unimportant as a sink of matter. Motivated by the inevitability of thermal disk-winds and wealth of data taken with current observatories such as Chandra, XMM-Newton and Suzaku, as well as the future AstroH mission, we decided to investigate the requirements to produce very dense winds. Using physical arguments, hydrodynamical simulations and absorption line calculations, we found that modification of the heating and cooling rates by a factor of a few results in an increase of the wind density of up to an order of magnitude and the wind velocity by a factor of about two. Therefore, the mass loss rate from the disk can be one, if not even two orders of magnitude higher than the accretion rate onto the central object. Such a high mass loss rate is expected to destabilize the disk and perhaps provides a mechanism for state change.