Radio Reappearance of V4641 Sgr

ATel #296; M. P. Rupen, A. J. Mioduszewski, V. Dhawan (NRAO)
on 5 Jul 2004; 22:59 UT
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Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

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Very Large Array (VLA) observations of the black hole candidate binary V4641 Sgr, currently active at optical wavelengths (see VSNet), reveal an on-going radio outburst. Data taken on 5 July 2004 from 07:47 to 09:32 UT gave clear detections of an optically thin, steep spectrum source (flux density going as frequency to the -0.7 power):

•  1.425 GHz: 12.50 +/- 1.22 mJy/beam
•  4.860 GHz:   6.56 +/- 0.31 mJy/beam
•  8.460 GHz:   4.45 +/- 0.09 mJy/beam
• 14.940 GHz:   2.09 +/- 0.18 mJy/beam
• 22.460 GHz:   1.96 +/- 0.16 mJy/beam
• 43.399 GHz:   1.23 +/- 0.31 mJy/beam

There is no indication of any spatial extension to the source, although the limits are rather poor since the VLA is currently in its most compact (D) configuration. Gaussian fits for instance give an upper limit of 3.6 arcsec (full-width at half-maximum) at 8.46 GHz, the most sensitive and reliable frequency band.

There is marginal evidence that the flux density decreased during these observations. Two independent measurements at 8.46 GHz for instance showed the source to be 5.28 +/- 0.38 mJy at MJD 53191.3259 and 3.64 +/- 0.29 mJy at MJD 53191.3797, a decrease of 1.64 +/- 0.48 mJy over 80 minutes. This combined with the optically-thin spectrum suggests we are seeing the decaying part of an earlier radio flare.

Our most recent pre-outburst observations were made at 8.46 GHz on 6 June 2004, yielding a non-detection with a nominal flux density of -0.026 +/- 0.083 mJy/bm at the location of the source.

Given the strong variability previously seen for this source, and the on-going optical activity, further flares at all wavelengths are quite likely. This strong and unpredictable variability makes truly simultaneous observations essential to understanding the nature of this unique system. Accordingly, we will post a schedule of all the upcoming observations we know about on the Web, at http://www.aoc.nrao.edu/~mrupen/XRT/V4641Sgr/v4641sgr.shtml. We would greatly appreciate advance notification of any additional observations that do not appear on this list, to aid other observers in planning their own simultaneous campaigns.

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Upcoming observations of V4641 Sgr