Enigmatic X-Ray Emission from Colliding wind binary WR 21a

Mass-loss is one of the most important and uncertain parameters in the evolution of a massive star. Colliding-wind binaries are ideal testing grounds for plasma-shock physics, mainly because plasma properties vary with binary separation in many cases. Here, we report The X-ray observations of the enigmatic W-R binary WR 21a. WR 21a is one of the X-ray brightest colliding wind binaries (e=0.64, P_orb=31.673 d). These masses are estimated to be M_WR > 87 solar masses (one of the most massive stars) and M_O > 53 solar masses. The first one-period monitoring performed by Swift/XRT in order to reveal the total variation in one period. Our observations cover 17 different epochs from 2013 October 1 to 2013 November 2 for a total exposure of about 73 ks. The X-ray spectra were well-fitted with single-absorbed component at kT ∼ 2-4 keV, which are compatible with the thermal shocked plasma. The absorption-corrected X-ray luminosity in the 0.5-10 keV band varies from 2e33 to 1e34 ergs s(-1) . It is found for the first time that the luminosity varies roughly in inverse proportion to the separation of the two stars before the X-ray maximum but later drops rapidly toward periastron. This discrepancy can be explained if the O-star wind collides with the W-R star wind before it has reached its terminal velocity, leading to a reduction in its wind momentum flux. In addition, using Chandra/ACIS archived data, we discovered a peculiar, flare-like X-ray variability around apastron. There is the potential for a flare-like increase of the mass-loss rate. In this paper, we will discuss the interpretations of these peculiar X-ray variations and also introduce the wind measurement using Astro-H.