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

X-ray binaries (XRBs) often exhibit a high energy power-law tail (HEP-tail) and these tails can be generated by bulk Comptonization (BMC) process with a free-fall bulk region onto the compact object. The radio emission (which is generated by synchrotron-emitting outflowing electrons) is also observed in all spectral states of XRBs. Interestingly, the variation of HEP-tail flux in 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 collimated and conical (of opening angle $\theta_b$ with axis perpendicular to the accretion disk) outflow geometries. 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 $\theta_b$ $\gtrsim$30 degree in conical outflow, while for either collimated or conical (with $\theta_b$ $<$ 30degree) outflow these HEP-tail can be only generated when it is also found in thermal Comptonized spectra (i.e., at sufficiently high medium temperature). We describe the observed GRS 1915+105 spectrum for two classes $\chi$ and $\gamma$ in conical outflow, in both cases the estimated outflow speed is relativistic and the kinetic power of wind indicates that the HEP-tail can be generated at inner region of the accretion disk, like inner disk radio emission.