  We report on the rapid X-ray variability of the variable star and X-ray
transient V4641 Sagittarii (SAX J1819.3-2525; XTE J1819-254) as observed on 15
Sept. 1999 by the proportional counter array (PCA) on board the Rossi X-ray
Timing Explorer (RXTE). During the first ~900 s of the first PCA observation,
V4641 Sgr showed very strong X-ray fluctuations by a factor of 4 on time scales
of seconds to about 500 on time scales of minutes. The spectrum of the source
during this flaring episode became harder when the count rate decreased. After
this flaring episode, V4641 Sgr entered a quiescent state in which it remained
for the rest of this, and the subsequent PCA observations. The X-ray spectrum
was considerably softer in this quiescent state than during the flaring
episode. The intrinsic X-ray luminosity (both during the flaring episode and
the quiescent state) and the rapid X-ray variability do not strongly constrain
the nature of the compact object (neutron star or black hole) in the system,
although a black hole seems to be more likely. The very short duration of the
bright X-ray phase of V4641 Sgr and its likely close distance suggest that many
similar objects could be present in our galaxy, most of which are not noticed
when they are in X-ray outburst due to the short duration of these outbursts. A
considerable number of the black holes present in our galaxy might be contained
in systems similar to V4641 Sgr.
