V4641 Sgr returns to quiescence; deep observations planned

ATel #315; M. P. Rupen (NRAO), P. G. Jonker (CfA), A. J. Mioduszewski, V. Dhawan (NRAO), R. P. Fender (U. Amsterdam), & G. Dubus (LLR)
on 6 Aug 2004; 17:58 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

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Observations of the X-ray binary V4641 Sgr (e.g., ATEL #309, ATEL #303) with the Very Large Array (VLA) show that the object has returned to quiescence. Our last clear detection was on 20 July 2004, when the 4.86 GHz flux density remained steady at 2.0 +/- 0.2 mJy in two five-minute observations centered on 05:06 and 08:53 UT. Data taken at the same frequency on 23 July 2004 gave a nominal flux density at the position of the source of 0.27 +/- 0.09 mJy/beam, while 8.46 GHz observations on 23, 27, 28, 31 July, 1 August gave nominal flux densities of 0.09 +/- 0.05, 0.06 +/- 0.08, 0.05 +/- 0.04, 0.105 +/- 0.034, and 0.00 +/- 0.10 mJy/beam, respectively.

A 20ksec observation with Chandra on July 30 detected 15 source counts (i.e., 8.75e-4 c/s). Although this is somewhat higher than the quiescent source count rate of 3e-4 c/s seen by Tomsick et al. (2003), this indicates that the source is also approaching X-ray quiescence.

These X-ray and radio observations are consistent with optical reports on VSNet (cf. http://www2.yukawa.kyoto-u.ac.jp/~uemura/v4641sgr/index.html ) which indicate strong activity on July 19/20, followed by a return to ordinary ellipsoidal variations by the 23rd. Note however that in previous years flares were observed for several months after the initial outburst.

In addition to on-going VLA monitoring, a simultaneous Chandra (40 ksec) + VLA (8 hour) observation is scheduled from 23:55 UT on 9 August 2004, to 07:54 UT on 10 August 2004. Simultaneous optical and near infrared observations are strongly encouraged.

V4641 Sgr radio data & upcoming observations