Monte Carlo Simulations of Thermal Comptonization Process in a Two Component Advective Flow around a Neutron Star

TL;DR

This study uses Monte Carlo simulations to analyze how thermal Comptonization in a two-component accretion flow around a neutron star affects spectral properties, revealing how accretion rates, boundary layer temperature, and inclination angle influence spectral hardness.

Contribution

It introduces a detailed Monte Carlo simulation framework for modeling spectral properties of neutron star accretion flows considering multiple physical parameters.

Findings

Spectral hardness increases with higher halo accretion rate.

Boundary layer temperature significantly influences spectral shape.

Inclination angle affects spectral hardening.

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.

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