  The black hole X-ray binary V4641 Sgr experienced an outburst in 2002 May
which was detected at X-ray, optical, and radio wavelengths. The outburst
lasted for only 6 days, but the object remained active for the next several
months. Here we report on the detailed properties of light curves during the
outburst and the post-outburst active phase. We reveal that rapid optical
variations of ~100 s became more prominent when a thermal flare weakened and
the optical spectrum flattened in the Ic, Rc, and V-band region. In conjunction
with the flat spectrum in the radio range, this strongly indicates that the
origin of rapid variations is not thermal emission, but synchrotron emission.
Just after the outburst, we detected repeated flares at optical and X-ray
wavelengths. The optical and X-ray light curves exhibited a strong correlation,
with the X-rays, lagging by about 7 min. The X-ray lag can be understood in
terms of a hot region propagating into the inner region of the accretion flow.
The short X-ray lag, however, requires modifications of this simple scenario to
account for the short propagation time. We also detected rapid optical
variations with surprisingly high amplitude 50 days after the outburst, which
we call optical flashes. During the most prominent optical flash, the object
brightened by 1.2 mag only within 30 s. The released energy indicates that the
emission source should be at the innermost region of the accretion flow.
