Low Frequency GMRT Observations of Microquasar V4641 Sgr

We have observed the micro-quasar V4641 Sgr (SAX J1819.3-2525) at the time of flare during May 2002, using the Giant Metrewave Radio Telescope at low radio frequencies (244 and 610 MHz). This is the lowest frequency radio detection of the source. The source showed clear signature of spectral evolution from optically thick to thin state. We model the spectral evolution of the source to obtain some of the source parameters. Assuming the value of magnetic field strength to be in the range of 0.1 to 1 Gauss, we obtain the initial velocity of expansion of the jet to be in the range $\sim$ 0.45c to 0.25c. This is consistent with the known expansion velocities of the jet with other microquasars.