  An optical spectroscopy of an unusual fast transient V4641 Sgr constrains its
mass to 8.7-11.7M_sun (9.6M_sun is the best fit value) and the distance to
7.4--12.3 kpc (Orosz et al. 2001). At this distance the peak flux of 12 Crab in
the 2--12 keV energy band, measured by ASM/RXTE, implies the X-ray luminosity
exceeding 2-3e39 erg/s, i.e. near or above the Eddington limit for a 9.6M_sun
black hole. An optical photometry shows that at the peak of the optical
outburst the visual magnitude increased by Delta m_V > 4.7^m relative to the
quiescent level and reached m_V < 8.8^m. An assumption that this optical
emission is due to irradiated surface of an accretion disk or a companion star
with the the black body shape of the spectrum would mean that the bolometric
luminosity of the system exceeds L>3e41 erg/s > 300 L_Edd.
  We argue that the optical data strongly suggest presence of an extended
envelope surrounding the source which absorbs primary X-rays flux and reemits
it in optical and UV. The data also suggests that this envelope should be
optically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgr
at the peak of an optical flare are very similar to those of SS433. This
envelope is likely the result of near or super Eddington rate of mass accretion
onto the black hole and it vanishes during subsequent evolution of the source
when apparent luminosity drops well below the Eddington value. Thus this
transient source provides us direct proof of the dramatic change in the
character of an accretion flow at the mass accretion rate near or above the
critical Eddington value as predicted long time ago by the theoretical models.
