| Abstract : | High-mass X-ray binary (HMXB) systems are co-rotating close binary stellar systems where the compact star is a neutron star or a black hole, and the companion star is a massive star with a mass typically greater than 10 solar mass. HMXBs are sources of strong X-ray radiation. Here we report detailed modelling of X-ray emission lines, Fe XXV (6.700 keV) and Fe XXVI (6.966 keV), from two stellar wind-fed HMXB systems, Cyg X--3 and 4U 1538--522, using the spectral synthesis code CLOUDY. We find that for Cyg X–3 and 4U 1538--522, the inner radius of the ionized gas is at a distance of 10$^{12.25}$ cm and 10$^{10.43}$ cm, respectively, from the primary star, which is the main source of ionization. The hydrogen number densities of the ionized gas for Cyg X--3 and 4U 1538--522 are $\sim$ 10$^{11.35}$ cm$^{-3}$ and 10$^{11.99}$ cm$^{-3}$, respectively. The corresponding wind velocities are 2000 km s$^{-1}$ and 1500 km s$^{-1}$. The respective predicted hydrogen column densities for Cyg X–3 and 4U 1538–522 are $10^{23.2}$ cm$^{-2}$ and 10$^{22.25}$ cm$^{-2}$. In addition, we find that the magnetic field affects the strength of the Fe XXV (6.700 keV) and Fe XXVI (6.966 keV) lines through cyclotron cooling. We estimate the most probable strength of the magnetic field for Cyg X–3 and 4U 1538–522, where the Fe XXV and Fe XXVI lines originate, is $\sim$ 10$^{2.5}$ G. |
