A dozen colliding-wind X-ray binaries in the star cluster R136 in the 30 Doradus region.
PORTEGIES ZWART S.F., POOLEY D. and LEWIN W.H.G.
Abstract (from CDS):
We analyzed archival Chandra X-ray observations of the central portion of the 30 Doradus region in the Large Magellanic Cloud. The image contains 20 X-ray point sources with luminosities between 5x1032 and 2x1035 ergs.s–1 (0.2-3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectral type O stars as optical counterparts. Nine of these are within ∼3.4 pc of R136, the central star cluster of NGC 2070. We derive an empirical relation between the X-ray luminosity and the parameters for the stellar wind of the optical counterpart. The relation gives good agreement for known colliding-wind binaries in the Milky Way Galaxy and for the identified X-ray sources in NGC 2070. We conclude that probably all identified X-ray sources in NGC 2070 are colliding-wind binaries and that they are not associated with compact objects. This conclusion contradicts earlier studies where it was argued, using ROSAT data, that two earlier discovered X-ray sources are accreting black hole binaries. Five of the 18 brightest stars in R136 are not visible in our X-ray observations. These stars either are single, have low-mass companions, or have very wide orbits. The resulting binary fraction among early-type stars is then unusually high (at least 70%).