Monte Carlo simulations of thermal comptonization process in a two-component advective flow around a neutron star

Abstract

We explore spectral properties of a Two-Component Advective Flow (TCAF) around a neutron star. We compute the effects of thermal Comptonization of soft photons emitted from a Keplerian disc and the boundary layer of the neutron star by the post-shock region of a sub-Keplerian flow, formed due to the centrifugal barrier. The shock location $X_s$ is also the inner edge of the Keplerian disc. We compute a series of realistic spectra assuming a set of electron temperatures of the post-shock region $T_{CE}$, the temperature of the normal boundary layer (NBOL) $T_{NS}$ of the neutron star and the shock location $X_s$. These parameters depend on the disc and halo accretion rates ($\dot{m_d}$ and $\dot{m_h}$, respectively) which control the resultant spectra. We find that the spectrum becomes harder when $\dot{m}_h$ is increased. The spectrum is controlled strongly by $T_{NS}$ due to its proximity to the Comptonizing cloud since photons emitted from the NBOL cool down the post-shock region very effectively. We also show the evidence of spectral hardening as the inclination angle of the disc is increased.