Outburst of the Galactic microquasar V4641 Sgr V4641 Sgr outburst!!! Rod Stubbings reports that the microquasar V4641 Sgr = SAX J1819.3-2525 (formerly called GM Sgr) is in outburst as indicated by the following observations. V4641 Sgr 020517.569 13.4 V4641 Sgr 020519.699 11.5 020519.704 11.5 020519.716 11.5 020519.723 11.5 The object underwent a giant outburst in 1999 September (Stubbings and Pearce (1999) IAUC 7253), preceded by optical brightening (Kato et al. (1999) IBVS 4777) which was first detected by T. Watanabe. [Excerpt from Uemura et al. (2002) PASJ 54, 95] Microquasars are a class of X-ray novae with relativistic radio jets. Their X-ray and optical emissions are thought to be from an accretion disk around a neutron star or a black hole, however their relationship with the jet production mechanism is poorly understood. Here we report on the unprecedented discovery of a giant optical outburst in the X-ray nova V4641 Sagittarii (= SAX J1819.3-2525 = XTE J1819-254), which enabled subsequent X-ray observations accurately pinpointing an intense X-ray flare reaching 12 Crab and establishing it as a microquasar at a small distance. The decay with an e-folding time of 0.2 day is the shortest ever observed for any X-ray novae, implying a new category. Observations revealed the first ever exemplification of the detailed optical behavior prior to the X-ray maximum, and moreover, the totally unexpected 7 hour precedence of the optical maximum to the X-ray maximum. The seemingly anti-correlated light curves in X-ray and optical can be best understood as a short episode of supercritical accretion producing a relativistic jet. X-ray novae are binary systems which exhibit luminous X-ray and optical outburst which lasts for a few tens of days. They uniquely provide the most compelling evidence for the existence of steller mass black holes using radial velocity studies, giving mass functions exceeding the maximum mass of a stable neutron star (~3 Msolar). Microquasars are X-ray novae with superluminal jets whose mechanism is poorly unknown while a number of observational results and models have been discussed. Supercritical accretion disks have recently been discussed for black hole candidates shining at the Eddington luminosity which frequently show jets. The accretion disks theoretically become geometrically and optically thick when the mass accrete over a critical rate, on the other hand, the geometrically thin disks are applied for the subcritical accretion rate. We can consider that supercritical accretion occurs in persistent jet sources, for example, active galactic nuclei or SS433 whose mass accretion rate is observationally suggested to be near critical. On the other hand, during any transient jet source outbursts, we had detected no implication that supercritical accretion occurs. However, a new atypical microquasar, V4641 Sgr showed an evidence of this for the first time.