ASCA observations of 2 Velorum (WC8 + O9I): the variable X-ray spectrum of colliding winds

Abstract

We present an analysis of two observations of the nearby Wolf—Rayet (WR) + O-star binary system γ2 Velorum with the Advanced Satellite for Cosmology and Astrophysics (ASCA). These observations were taken at orbital phases (ϕ = 0.4 and 0.5, when the O star is close to being in front of the WR star. At phase ϕ = 0.5, γ2 Vel is approximately four times more luminous in X-rays than it is at ϕ = 0.4. We fit the X-ray spectra using simple combinations of absorbed Raymond—Smith emission models. We find evidence of a cool (kT∼ 03 keV) and a hot (kT∼ 13 keV) component in each spectrum. The hotter component suffers from variable absorption which suggests that it originates in a localized region between the two stars. Between ϕ = 0.4 and 0.5 the absorbing column for this hotter component drops by a factor of 3 which accounts for most of the corresponding increase in flux. These observations are interpreted in terms of a colliding-wind model. The collision between the winds from the O star and the WR star generates a region of hot X-ray-emitting gas. When the WR star is in front, the dense WR wind effectively absorbs all the X-ray flux from the wind collision. However, when the O star is in front the more diffuse wind of the O star allows substantially more flux from the wind collision to escape. Our ASCA data confirm the colliding-wind origin for the hard X-ray emission from γ2 Vel, first identified from ROSAT data by Willis, Schild & Stevens. At ϕ = 0.5 the emergent luminosity is Lx ∼ 3.9 × 1032 erg s−1. We also use hydrodynamic models of colliding winds to calculate synthetic X-ray spectra for γ2 Vel, which we then fit to the ASCA spectra. This allows us to determine fundamental stellar wind parameters for both stellar components from the X-ray spectra. In particular, we determine a mass loss for the WC8 star of ∼ 3 × 10−5 M⊙ yr−1, a value 3 times lower than that determined by radio observations. The consequences of this are discussed. We consider that we are definitely seeing colliding-wind emission and also show the great potential for X-ray spectroscopy in determining fundamental wind parameters of WR (and other hot luminous) stars.