Note that the recently active object that was once identified as GM Sgr is another star which is now called V4641 Sgr IAUC 7277)
Image of V4641 Sgr (=SAX J1819.3-2525) made from 4.9 GHz VLA A-array data on Sep 16.02 UT. The total flux in this image is 0.41 Jy. This was part of the data showing that the central radio source at R.A. = 18h19m21.637s, Decl. = -25o24'25.6", as reported in IAU Circular 7254, coincides with the position of V4641 Sgr (=USNO0600.03786) which is R.A. = 18h19m21.64s, Decl. = -25o24'25.7". The two extensions are about 0.3" North and South. ATCA observations that began shortly after the Sept. 16.02 VLA observations showed the radio source was decreasing by about a factor of two in 12 hours at the higher frequencies, as reported in IAUC 7256. The original discovery of SAX J1819.3-2525 in March 1999 was reported in IAUC 7119. The sudden appearance of very strong X-ray emission, up to 12 Crab units for the RXTE ASM and 8 Crab units for CGRO BATSE was reported in IAUC 7253 and IAUC 7258.
In this image the N-S beam elongation has been removed by restoring the image with a small symmetrical beam. The most objective evidence that the radio source is resolved on Sept. 16.02 UT is seen in the visibility amplitudes which decrease with increasing baseline length.
Radio light curves for V4641 Sgr showing the data from the VLA, ATCA (Bryan Gaensler, private communiction), and GBI. As indicated in the figure a power law decay with spectal index ~ -0.66 and power law index -2.05 is roughly consistent with all these data.
The distance to V4641 Sgr seems to be between 0.4 and 1 kpc. Jochen Greiner (private communication) used optical spectrum information to estimate a distance between 0.4 and 1.2 kpc. We were able to do an HI absorption experiment against the radio source and this seems to indicate a distance of less than 1 kpc.
A web page maintained by Don Smith for the RXTE ASM team contains more detailed X-ray oriented information about V4641. An optically oriented web page is maintained by variable star observers at Kyoto University.
R.M. Hjellming (NRAO), M.P. Rupen (NRAO), and A.J. Mioduszewski (Sydney University)
For more information contact R.M. Hjellming