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

TL;DR

This study models soft X-ray emission lines from high-mass X-ray binaries Cyg X-3 and 4U 1538-522 using CLOUDY, estimating physical parameters and the influence of magnetic fields on spectral lines.

Contribution

It provides detailed physical parameter estimates for two HMXBs and explores the impact of magnetic fields on their X-ray spectral lines, which is a novel aspect.

Findings

Inner radius of ionised gas: 10^12.25 cm (Cyg X-3), 10^10.43 cm (4U 1538-522)

Densities of ionised gas: ~10^11.35 cm^-3 (Cyg X-3), 10^11.99 cm^-3 (4U 1538-522)

Magnetic field strength estimated at ~10^2.5 G for both sources.

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.