Multiwavelength studies of WR 21a and its surroundings

Abstract

We present results of high-resolution radio continuum observations towards the binary star WR 21a (Wack 2134) obtained with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.64 GHz. We detected the system at 4.8 GHz (6 cm) with a flux density of 0.25+-0.06 mJy and set an upper limit of 0.3 mJy at 8.64 GHz (3 cm). The derived spectral index of alpha < 0.3 [S(nu)=k.nu**(alpha)] suggests the presence of non-thermal emission, probably originating in a colliding-wind region. A second, unrelated radio source was detected ~10" north of WR 21a at (RA, Dec)(J2000)=(10h 25m 56.49, -57deg 48' 34.4", with flux densities of 0.36 and 0.55 mJy at 4.8 and 8.64 GHz, respectively, resulting in alpha = 0.72. HI observations in the area are dominated by absorption against the prominent HII region RCW 49. Analysis of a complete set of archived X-ray observations of WR 21a confirms its strong variability but throws into doubt previous suggestions by Reig (1999) of a period of years for the system. Finally, we comment on the association with the nearby EGRET source 3EG J1027-5817.

9 pages, 5 figures, accepted (Astronomy & Astrophysics)

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• The Arches cluster revisited: II. A massive eclipsing spectroscopic binary in the Arches cluster
• The X-ray light curve of the massive colliding wind Wolf-Rayet + O binary WR21a
• The Massive Star-Forming Regions Omnibus X-Ray Catalog, Third Installment
• The binary nature of the Galactic Centre X-ray source CXOGC J174536.1-285638
• Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II: Spectroscopic and Environmental Analysis of Thorne-Żytkow Object and Super-AGB Star Candidates
• The Arches cluster revisited: IV. Observational constraints on the binary properties of very massive stars
• Infrared dust bubble CS51 and its interaction with the surrounding interstellar medium
• Wolf-Rayet stars, black holes and the first detected gravitational wave source
• Gamma rays from colliding winds of massive stars