2020RAA....20..108R

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars. In our two previous papers, we study the high-resolution X-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters. We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field, mass loss rate and terminal wind velocity. Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent. In the present paper, we analyzed low-resolution X-ray spectra, using model-independent X-ray hardness values for checking the above mentioned dependencies. We establish that X-ray luminosities L_X weakly depend on the stellar magnetic field. At the same time, $L_X ∝{dot}M^{0.5}$ and $L_X ∝E^{0.5}_kin$, where ${dot}M$ is the mass loss rate and $E_kin$ is the kinetic energy of the wind. The X-ray luminosities decrease with growing magnetic confinement parameter η. We also argue that there is an additional (probably non-thermal) component contributed to the stellar X-ray emission.