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e-VLBI detection of Circinus X-1 at 1.6 and 8.4 GHz

ATEL # 1037; A. Deller (Swinburne University of Technology), C. Phillips (CSIRO ATNF), S.W. Amy (CSIRO ATNF), A.K. Tzioumis (CSIRO ATNF), J.E. Reynolds (CSIRO ATNF), D.L. Jauncey (CSIRO ATNF), J. Stevens (University of Tasmania), S. Ellingsen (University of Tasmania), J. Dickey (University of Tasmania), R. Fender (University of Southampton), G.D. Nicolson (Hartebeestoek Radio Astronomy Observatory), S.J. Tingay (Swinburne University of Technology), V. Tudose (University of Amsterdam)
on 30 Mar 2007; 23:58 UT
Password Certification: Steven Tingay (

Subjects: Radio, Binaries, Neutron Stars, Transients

Following the return of Circinus X-1 to a recurrent radio flaring state (last seen in 1975 - 1985: ATel #563, #985) we have performed e-VLBI observations on the source at 1.6 and 8.4 GHz using an Australian array of radio telescopes (Parkes, ATCA, Mopra, and Hobart). e-VLBI allows rapid response to transient events and real-time analysis of VLBI data, in this case implemented by transfer of data between telescopes via optical fibres and software correlation on a PC cluster. Observations were made on 2007 March 23/24/25, commencing approximately 10 hours after the predicted peak of a radio flare, and reveal a 10 mJy source of extent 60 mas (best-fit circular Gaussian FWHM) at 1.6 GHz. At 8.4 GHz the source has a flux density of 5 mJy and was unresolved on the ATCA - Mopra baseline (~100 km ~ 60 mas). However, it was not detected on the long baselines to Hobart (~1500 km ~ 5 mas) due to the limited sensitivity. This implies an angular size significantly less than 60 mas at 8.4 GHz, consistent with scatter broadening by the interstellar medium. The source was observed over two consecutive nights at 1.6 GHz but no significant variation in the flux density was observed in this 24 hour period at this frequency. Continuing total flux density observations at Hartebeesthoek Radio Astronomy Observatory have shown that the strong flaring observed at 8.5 and 4.8 GHz (ATel #985) has declined rapidly during 2007 with the source currently below the 30 mJy detection level at Hartebeesthoek. This matches the very low X-ray levels recently observed by RXTE.

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