2020ApJ...891..104A

An X-ray study of the deeply embedded Wolf-Rayet star WR 121a has been carried out using long-term (spanning over ∼12 yr) archival observations from Chandra and XMM-Newton. For the first time, a periodic variation of 4.1 days has been detected in the X-ray light curve of WR 121a. No companion is seen in a merged and exposure-corrected Chandra X-ray image of WR 121a, as found in other previous observations in the J band. The X-ray spectrum of WR 121a is well explained by a thermal plasma emission model with temperatures of 0.98 ± 0.34 keV and 3.55 ± 0.69 keV for the cool and hot components, respectively, and nonsolar abundances. The present study indicates that WR 121a is an X-ray-bright massive binary with an X-ray luminosity of ∼10³⁴ erg s^–1, which can be explained by active wind collision between its components. Phase-locked modulations have been seen in the flux variation of WR 121a where the flux increases by a factor of ∼1.6 from minimum to maximum in the 0.3-10.0 keV energy band. These variations could be caused by the wind collision region being eclipsed by the secondary star in a binary orbit. The winds of both components of WR 121a appear to be radiative. Radiative inhibition and radiative braking are the most likely processes that are severely affecting the wind collision in this short-period massive binary system.