High energy power-law tail in X-ray binaries and bulk Comptonization due to a outflow from a disk

Abstract

We study the high energy power-law tail emission of X-ray binaries (XRBs) by a bulk Comptonization process which usually observe in the very high soft VHS state of blackhole BH XRBs and the high soft HS state of neutron star (NS) and BH XRBs. Earlier, to generate the power-law tail in bulk Comptonization framework, a free-fall converging flow into BH or NS has been considered as a bulk region. In this work, for a bulk region we consider mainly an outflow geometry from the accretion disk which is bounded by a torus surrounding the compact object. We have a two choice for an outflow geometry i) collimated flow ii) conical flow of opening angle $\theta_b$ and the axis is perpendicular to the disk. We also consider an azimuthal velocity of the torus fluids as a bulk motion where the fluids are rotating around the compact object (a torus flow). We find that the power-law tail can be generated in a torus flow with having large optical depth and bulk speed ($>$ 0.75c), and in conical flow with $\theta_b$ $>$ $\sim$30 degree for a low value of Comptonizing medium temperature. Particularly, in conical flow the low opening angle is more favourable to generate the power-law tail in both state HS and VHS. We notice that when the outflow is collimated, then the emergent spectrum does not have power-law component for a low Comptonizing medium temperature.Comment: 6 pages, 3 figures. This work was presented in RETCO-III, June 5-7, 2017, held at IIST, Thiruvananthapuram, Kerala (India). Appear in the Journal of Astrophysics and Astronomy (as a Special Issue