Expected evolution of disk wind properties along an X-ray binary outburst

Abstract

Blueshifted X-ray absorption lines (preferentially from Fe XXV and Fe XXVI present in the 6–8 keV range) indicating the presence of massive hot disk winds in black hole (BH) X-ray binaries (XrB) are most generally observed during soft states. It has been recently suggested that the nondetection of such hot wind signatures in hard states could be due to the thermal instability of the wind in the ionization domain consistent with Fe XXV and Fe XXVI. Studying the wind thermal stability does require, however, a very good knowledge of the spectral shape of the ionizing spectral energy distribution (SED). In this paper, we discuss the expected evolution of the disk wind properties during an entire outburst by using the RXTE observations of GX 339-4 during its 2010–2011 outburst. While GX 339-4 never showed signatures of a hot wind in the X-rays, the dataset used is optimal for the analysis shown in this study. We computed the corresponding stability curves of the wind using the SED obtained with the jet-emitting disk model. We show that the disk wind can transit from stable to unstable states for Fe XXV and Fe XXVI ions on a day timescale. While the absence of wind absorption features in hard states could be explained by this instability, their presence in soft states seems to require changes in the wind properties (e.g., density) during the spectral transitions between hard and soft states. We propose that these changes could be partly due to the variation of the heating power release at the accretion disk surface through irradiation by the central X-ray source. The evolution of the disk wind properties discussed in this paper could be confirmed through the daily monitoring of the spectral transition of a high-inclination BH XrB.