Abstract
Context. Massive star winds are known to be responsible for X-ray emission arising from wind plasma heated by the strong shocks up to temperatures of 106–107 K in the case of colliding wind binaries. The investigation of X-ray emission from massive stars thus constitutes a valuable tool for identifying binaries, which is otherwise a difficult task using classical techniques.
Aims. We investigated thermal and nonthermal X-ray emission from the massive O-type star HD 93250 to unveil its binary orbital parameters independently.
Methods. To meet our goal, we analyzed X-ray data obtained with European Photon Imaging Camera on board XMM-Newton spanning over ~19 yr. Additionally, we analyzed NuSTAR observations of HD 93250 taken at various epochs.
Results. We determine the variability timescale of the X-ray emission to be 193.8±1.3d, in full agreement with the 194.3±0.4d period derived from the astrometric orbit. The X-ray spectrum of HD 93250 is well explained by a three-temperature thermal plasma emission model with temperatures of 0.26, 1.0, and 3.3 keV. The resulting X-ray flux varies in compliance with the typical colliding wind emission from eccentric massive binaries where it enhances near periastron passage and decreases gradually close to apastron in proportion to the inverse of the binary separation. The periastron-to-apastron X-ray emission ratio points to an eccentricity range of 0.20–0.25, once again in agreement with the previously determined astrometric orbit. Finally, we do not detect any hard X-ray emission attributable to nonthermal emission above 10 keV.
Conclusions. Given the derived plasma temperature, the strong phase-locked variability, and the significant over-luminosity in X-rays, we establish that the X-ray emission from HD 93250 is dominated by the colliding-wind region. Our results lend support to the idea that X-ray time analysis of massive stars constitutes a relevant tool for investigating their multiplicity and for extracting relevant information on their basic orbital parameters – such as period and eccentricity – independently of any orbital solution derived from classical techniques.