Abstract Details

Name: Susmita Chakravorty
Affiliation: Indian Institute of Science
Conference ID: ASI2017_1349
Title : MHD acceleration for accretion disk winds around black hole binaries
Authors and Co-Authors : Susmita Chakravorty (IISc, India), Pierre-Olivier Petrucci (IPAG, Grenoble, France), Jonathan Ferreira (IPAG, Grenoble, France)
Abstract Type : Oral
Abstract Category : Stars,ISM and the Galaxy
Abstract : 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.