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An optical spectroscopy of an unusual fast transient V4641 Sgr constrains itsmass to 8.7-11.7M_sun (9.6M_sun is the best fit value) and the distance to7.4--12.3 kpc (Orosz et al. 2001). At this distance the peak flux of 12 Crab inthe 2--12 keV energy band, measured by ASM/RXTE, implies the X-ray luminosityexceeding 2-3e39 erg/s, i.e. near or above the Eddington limit for a 9.6M_sunblack hole. An optical photometry shows that at the peak of the opticaloutburst the visual magnitude increased by Delta m_V > 4.7^m relative to thequiescent level and reached m_V < 8.8^m. An assumption that this opticalemission is due to irradiated surface of an accretion disk or a companion starwith the the black body shape of the spectrum would mean that the bolometricluminosity of the system exceeds L>3e41 erg/s > 300 L_Edd.  We argue that the optical data strongly suggest presence of an extendedenvelope surrounding the source which absorbs primary X-rays flux and reemitsit in optical and UV. The data also suggests that this envelope should beoptically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgrat the peak of an optical flare are very similar to those of SS433. Thisenvelope is likely the result of near or super Eddington rate of mass accretiononto the black hole and it vanishes during subsequent evolution of the sourcewhen apparent luminosity drops well below the Eddington value. Thus thistransient source provides us direct proof of the dramatic change in thecharacter of an accretion flow at the mass accretion rate near or above thecritical Eddington value as predicted long time ago by the theoretical models.Comment: 4 pages, 2 figures. Submitted to A&A Letter

V4641Sgr – A super-Eddington source enshrouded by an extended envelope