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IAUC 8180: V4641 Sgr

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                                                  Circular No. 8180
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7440/7244/7444 (for emergency use only)

     M. P. Rupen, A. J. Mioduszewski, and V. Dhawan, National Radio
Astronomy Observatory, report the detection of renewed radio
activity in the x-ray transient and blackhole candidate V4641 Sgr
(= SAX J1819.3-2525).  Very Large Array (VLA) 4.86-GHz observations
on July 31 UT showed no source (rms noise level 0.1 mJy/beam),
while data taken on Aug. 6 gave strong detections with roughly
linear declines at both 4.86 (over 14 min, from 664 +/- 2 mJy at
Aug. 6.142 to 621 +/- 2 mJy at Aug. 6.151) and 8.46 GHz (over 4
min, from 569 +/- 2 mJy at Aug. 6.152 to 545 +/- 2 mJy at Aug.
6.155).  Extrapolating linear fits to both decays gives a spectral
index, alpha (S_nu going as nu**alpha), of about -0.14 at Aug.
6.1515, suggesting optically thick synchrotron radiation.  Further
VLA observations on Aug. 8, 9, and 10 (lasting from a few min to
1.5 hr) show a much fainter source, with flux densities ranging
from 2 to 8 mJy at 4.86 GHz, and a consistent, optically thin
spectral index of about -0.6 between 4.86 and 8.46 GHz.  These data
seem to be sampling a series of radio flares, as the flux density
changes abruptly from day to day and within each run; on Aug. 10,
the flux density declines by a factor of two over about 90 min.
VLA observations on Aug. 11 give a marginal detection at 8.46 GHz
of 0.34 +/- 0.08 mJy and a non-detection at 4.86 GHz (3-sigma upper
limit of 0.71 mJy).  This behavior is reminiscent of that in 2002
(IAUC 7906, 7908, 7928), in which an initial very strong flare was
followed by two months of continuing activity, characterized by
rapid low-level flaring at optical, x-ray, and radio wavelengths.
The detections, made with the VLA in its most extended (A)
configuration, provide an improved radio position for the source:
R.A. = 18h19m21s.634, Decl. = -25o24'25".84 [equinox 2000.0; via
the nearby astrometric calibrator ICRF J182057.8-252812 (Ma et al.
1998, A.J. 116, 516); uncertainty dominated by systematic errors
(rms perhaps as large as +/- 0".030).  There is no sign of any
extension in these images, which have a typical beam size at 8.46
GHz of 0".45 x 0".20 (FWHM of fitted Gaussian), oriented roughly
north/south.  D. Campbell-Wilson and R. Hunstead (University of
Syndey) further report a 4.5 +/- 0.8 mJy radio detection with the
Molonglo Observatory Synthesis Telescope at 843 MHz, in an 8-hr
observation centered on Aug. 8.465 -- consistent with the VLA
observations (above).  Further radio observations are planned, and
observations at other wavelengths are urgently requested.
Simultaneous multi-wavelength observations are particularly
valuable; a listing of known future observations is given at

                      (C) Copyright 2003 CBAT
2003 August 11                 (8180)            Daniel W. E. Green

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