e-VLBI observations of Cyg X-3

ATel #1476; V. Tudose (UvA), Z. Paragi (JIVE), R. Fender (Soton), R. Spencer (JBO), M. Garrett (ASTRON), A. Rushton (JBO)
on 15 Apr 2008; 10:04 UT
Credential Certification: Zsolt Paragi (zparagi@jive.nl)

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We observed the X-ray binary Cyg X-3 on April 9th, 2008 for 9.5 hours, between 03:30-13:00 UT, at 5 GHz with the European VLBI Network (EVN) in e-VLBI mode (the data from the radio telescopes are sent over optical fibers in real-time to the correlator for processing). The radio telescopes participating in the experiment were: Cambridge, Medicina, Jodrell Bank MkII, Onsala (25 m), Torun and Westerbork (phased array).

In the last few weeks Cyg X-3 has been passing through a clear transition towards softer states, as indicated by the ASM detector onboard the Rossi X-ray Timing Explorer (RXTE). This kind of behaviour has been associated in previous observations of Cyg X-3, as well as other X-ray binaries, with intermittent relativistic ejections of matter, detectable at radio wavelengths. We therefore triggered the e-VLBI observations.

The preliminary analysis reveals that the target is resolved and appears as two distinct compact emitting regions, with total flux density around 1.3 mJy (rms noise 0.09 mJy/beam). Both of these regions have been identified in one or more previous e-EVN images (Tudose et al. 2007 MNRAS 375, L11), but their nature is still not known. However, it is clear that Cyg X-3 shows structural changes on milliarcsecond scales even when the source does not show significant radio flaring activity. It is interesting to note that the radio "core" of the system (whose position was identified by Mioduszewski et al. 2001 ApJ 553, 766 and Miller-Jones et al. 2004 ApJ 600, 368) was not detected in any of the e-EVN observations in 2006-2008.

e-VLBI developments in Europe are supported by the EC DG-INFSO funded Communication Network Development project 'EXPRES', contract No. 02662. The European VLBI Network is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils.

We thank the EVN-PC for its support for this triggered e-VLBI observation and the JIVE staff (particularly R. Campbell) for their efforts in assuring the success of the experiment.