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

TL;DR

This study models the high energy power-law tail in X-ray binary spectra as a result of bulk Comptonization in conical outflows, revealing conditions under which such tails form and linking them to observed radio emissions.

Contribution

It introduces a Monte Carlo simulation of bulk Comptonization in conical outflows, showing how outflow geometry influences high energy tail formation in X-ray binaries.

Findings

Power-law tails form for outflow angles >30 degrees

High-energy tails require high medium temperatures in collimated outflows

Results align with analytical predictions and explain observed spectra of GRS 1915+105

Abstract

X-ray binaries (XRBs) often exhibit a high energy power-law tail (HEP-tail) and these tails can be generated by the bulk Comptonization (BMC) process with a free-fall bulk region onto the compact object. The radio emission (which is generated by a synchrotron-emitting outflowing electrons) is observed in all spectral state of XRBs. Interestingly, the variations of HEP-tail flux among different spectral states is similar to the variation of radio flux. We motivate to study the HEP-tail in BMC process with an outflowing medium. For this we consider a collimated and conical (of opening angle $\theta_b$ with axis perpendicular to the accretion disk) outflow geometry. We simulate the BMC spectrum by using a Monte Carlo scheme. We find that the emergent spectrum has power-law tail (of photon index $\Gamma$ $>$ 2 and with high energy cut-off $E_{c}$ $>$ 200 keV) only for $\theta_b$ greater than $\sim$30 degrees in conical outflow, while for a collimated or a conical outflow ($\theta_b$ $<$ 30 degrees) these HEP-tail can be only generated when it is also found in thermal Comptonized spectra (i.e., at sufficiently high Comptonizing medium temperature). These results are approximately consistent with analytically derived expressions. We describe the observed GRS 1915+105 spectrum for two classes $\chi$ and $\gamma$ in conical outflow, for this the outflow speed is highly relativistic and the kinetic power of wind suggest that the HEP-tail can be generated at inner region of the accretion disk, like inner disk radio emission.