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Strong radio flare in V4641 Sgr (= SAX J1819.3-2525)

ATel #172; M. P. Rupen, A. J. Mioduszewski, & V. Dhawan (NRAO)
on 7 Aug 2003; 14:04 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

Subjects: Radio, Infra-Red, Optical, X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 175, 599

We report a strong radio flare from the X-ray transient and likely black hole binary V4641 Sgr, which has recently become active again in the optical (ATEL #170) and X-ray (ATEL #171) bands. Observations with the Very Large Array (VLA) on August 6, 2003 gave strong detections at both 4.86 and 8.46 GHz, compared with a non-detection at 4.86 GHz on July 31, 2003 with an rms noise level of 0.1 mJy/beam. The August 6 data show a roughly linear decline at 4.86 GHz over 14 minutes, from 664+/-2 mJy at MJD 52857.142, to 621+/-2 mJy at MJD 52857.151; and a similarly linear decay over 4 minutes at 8.46 GHz, from 569+/-2 mJy at MJD 52857.152, to 545+/-2 mJy at MJD 52857.155. Extrapolating linear fits to both decays, this gives a spectral index alpha (S_nu going as nu^alpha) of about -0.14 at MJD 52857.1515, suggesting optically-thick synchrotron radiation.

These VLA A configuration images give an improved radio position for the source (J2000) of

18 19 21.634    -25 24 25.845
referenced to the VLBI position of the nearby (27arcmin) calibrator J1820-2528. This is likely dominated by systematic rather than statistical uncertainties, with an rms of perhaps +/-30 mas. The 8.46 GHz images were made with a resolution of 0.46 x 0.20 arcseconds (full-width at half-maximum of a Gaussian fit, at a position angle of -11 degrees), and there is no sign of any extension in these preliminary total intensity images.