Understanding Disc Winds in X-ray Binaries

High inclination X-ray binaries show blueshifted ionised absorption lines from disc winds, whose launching mechanism is still in debate.The lines are mainly observed in the soft state, where the continuum spectrum is dominated by the disc blackbody component, while disappear in the hard state, where the spectrum is characterized by a hard power-law component. This state-dependence is opposite to that of jets, and thus invoked an idea that the winds are magnetically powered like jets. However, there are also many previous studies suggesting that the observed lines can be interpreted by the thermal wind, which is driven by the irradiation of outer disc by the X-rays from the inner disc. In this talk, we overview current understanding of the observed properties of the winds and their launching mechanism. We then present our recent results of the direct comparison between the prediction of the thermal wind model and the observed line features of the black hole X-ray binary H 1743-322. We adopted a simple analytic thermal wind model to derive the column density, ionisation parameter, and velocity of the wind given the broadband continuum shape and luminosity determined from the RXTE and Swift data. Using these parameters we performed photoionised plasma simulations to predict the detailed line features at epochs where Chandra high-resolution spectroscopy was performed. The model was found to well reproduce the observed lines in the soft state, and it also successfully predicted their disappearance in the hard state. These results indicate that the evolution of observed wind properties in different states during outbursts can be explained by the thermal wind model.