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.

Accepted for publication in MNRAS. 10 pages, 8 figures and 10 tables

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