A comprehensive X-ray analysis of the massive O-type binary HD93250 over two decades

Massive star winds are known to be responsible for X-ray emission arising
from wind plasma heated by the strong shocks up to the temperature of
10$^6$--10$^7$ K in case of colliding wind binaries. We have investigated
thermal and non-thermal X-ray emission from the massive O-type star HD93250 to
unveil its binary orbital parameters independently. To meet our goal, X-ray
data obtained with XMM-Newton has been analyzed, spanning over $\sim$19 years.
Additionally, we analyzed NuSTAR observations of HD93250 taken at various
epochs. We determined the variability time-scale of the X-ray emission to be
193.8$\pm$1.3\,d, in full agreement with the 194.3$\pm$0.4\,d period derived
from the astrometric orbit. The X-ray spectrum of HD93250 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,
proportionally with 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 did not detect any hard X-ray emission attributable to
non-thermal emission above 10 keV. 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 HD93250 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 to investigate their
multiplicity and extract relevant information on their basic orbital
parameters, such as the period and the eccentricity, independently of any
orbital solution derived from usual techniques.