Magneto centrifugal winds from accretion discs around black hole binaries

High resolution X-ray spectra of black hole X-ray binaries (BHBs) show blueshifted absorption

lines suggesting the presence of outflowing winds. Furthermore, observations show that the disk

winds are equatorial and they occur in the Softer (disk dominated) states of the outburst and are

less prominent or absent in the Harder (power-law dominated) states. We are testing if self-

similar magneto-hydrodynamic (MHD) accretion-ejection models can explain the observational

results for accretion disk winds in BHBs. In our models, the density at the base of the outflow,

from the accretion disk, is not a free parameter, but is determined by solving the full set of

dynamical MHD equations without neglecting any physical term. Different MHD solutions were

generated for different values of (a) the disk aspect ratio and (b) the ejection efficiency `p’. We

generated two kinds of MHD solutions depending on the absence (cold solution) or presence

(warm solution) of heating at the disk surface. The cold MHD solutions are found to be

inadequate to account for winds due to their low ejection efficiency. The warm solutions can

have sufficiently high values of p (>= 0.1) which is required to explain the observed physical

quantities in the wind. The heating (required at the disk surface for the warm solutions) could

come from (i) the dissipation of energy due to MHD turbulence in the disk or (ii) from the

illumination of the disk surface which would be more efficient in the Soft state. We found that in

the Hard state a range of ionisation parameters is thermodynamically unstable, which makes it

impossible to have any wind at all, in the Hard state. Thus, using the MHD outflow models we

are able to explain the observed trends, i.e. that the winds are equatorial and that they are

observable in the Soft states (and not expected in the Hard state) of the BHB outbursts.

Encouraged by the success of the models we are formalising methods to predict theoretical high

resolution spectra to be fitted to absorption line observations from XMM-Newton and Chandra

gratings. Our models will include the key physical parameter p (the ejection efficiency) of the

accretion disk. Hence we hope to directly constrain physical parameters of the disk by fitting the observed spectra.