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

Journal of Astrophysics and Astronomy

NAGENDRA KUMAR^1¹ 

[1] Department of Physics, Indian Institute of Science, Bangalore 560 012, India.^1

关键词: Stars: black holes—stars: neutron—X-rays: binaries—X-rays: radiation mechanisms: thermal.;   

DOI  :  

学科分类：天文学(综合)

来源: Indian Academy of Sciences

【 摘 要 】

We study the high energy power-law tail emission of X-ray binaries (XRBs) by a bulk Comptonization process which is usually observed in the very high soft (VHS) state of black hole (BH) XRBs and the high soft (HS) state of the 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 had 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 two choices for an outflow geometry: (i) collimated flowand (ii) conical flow of opening angle $\theta_b$ and the axis is perpendicular to the disk. We also consider an azimuthalvelocity 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 having large optical depth and bulk speed (>0.75$c$), and in conical flow with $\theta_b$> $\sim 30^{\circ}$ 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 theHS state and the VHS state. We notice that when the outflow is collimated, then the emergent spectrum does not have power-law component for a low Comptonizing medium temperature.

【 授权许可】

CC BY