Analysis of the X-ray emission from OB stars III: low-resolution spectra of OB stars

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-resolutionX-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 proportional to E-kin(0.5) and L-X proportional to M-0.5, where 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 eta. We also argue that there is an additional (probably non-thermal) component contributed to the stellar X-ray emission.