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WR140 radio variation
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The significance of colliding stellar winds in massive binaries was first recognized for X-ray production, with the suggestion [1] that collision of a WR stellar wind with that from a companion OB star could be a significantly stronger source of X-rays than collision of the WR wind with the OB star itself . A survey of X-ray emission from WR stars [2] observed with EINSTEIN confirmed that binaries incorporating WR stars were stronger X-ray sources than single WR stars. As it happened, WR 140 was not observed with EINSTEIN but it has been observed with most other X-ray missions. Its X-ray emission was found to vary with the orbit, suffering highest extinction at the time of periastron passage, indicating movement of the X-ray source deep into the WR wind, consistent with its originating between the two stars [3], [9]. Colliding winds and X-ray production by WR 140 have been modelled both analytically [4] and numerically [5]. The non-thermal radio emission from WR140 varies similarly, being extinguished around periastron.

For direct observation of non-thermal radio emission and other colliding-wind phenomena between two luminous stars, we need to look to a wide colliding-wind WR system, like WR 147, whose radio and stellar components have been spatially resolved [6] as below. The X-ray emission from WR 147 arises [7] from the same region as the non-thermal radio emission, which has since been modelled theoretically [8].

Right: Direct evidence of a colliding-wind origin for non-thermal radio emission from the wide binary WR 147 (AS431). The radio emission is double, as can be seen in this contour map made with MERLIN. The separation of the radio sources is slightly less than that of the stars [6]. The southern radio source is the stellar wind emission from the WN8 star, which is evidently rather clumpy. The northern radio source is non-thermal emission from the region where the WN8 stellar wind collides with that of the OB star. This occurs between the two stars and is closer to the OB star because it has a weaker stellar wind than the WN8 star. The collision region is clearly elongated East-West. {Radio contours over stellar images}
rev. 25 Feb 2016
Peredur Williams
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[2] A.M.T. Pollock, ApJ., 320, 283, 1987
[3] P.M. Williams et al. MNRAS 243, 662, 1990
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[9] Y. Suguwara et al. PASJ, 67, 121, 2015