Spectroscopic modelling of two high-mass X-ray binaries, Cyg X--3 and 4U 1538--522

Abstract

We report a detailed modelling of soft X-ray emission lines from two stellar-wind fed Galactic high mass X-ray binary (HMXB) systems, Cyg X-3 and 4U 1538-522, and estimate physical parameters, e.g., hydrogen density, radiation field, chemical abundances, wind velocity, etc. The spectral synthesis code CLOUDY is utilized for this modelling. We model highly ionised X-ray spectral lines such as Fe XXV (6.700 keV), Fe XXVI (6.966 keV), and reproduce the observed line flux values. We find that for Cyg X--3 and 4U 1538-522, the inner radius of the ionised 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 ionisation. The densities of the ionised gas for Cyg X--3 and 4U 1538--522 are found to be $\sim$ 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 a magnetic field affects the strength of the spectral lines through cyclotron cooling. Hence, we perform separate model comparisons including a magnetic field for both the sources. Most of the parameters, except the hydrogen column density, have similar values with and without a magnetic field. We estimate that 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.