  X-ray pulses with millisecond-long FWHM have been detected in RXTE (Rossi
X-Ray Timing Explorer) observations of Cyg X-1. Their identity as
short-timescale variations in the X-ray luminosity of the source, and not
stochastic variability in the X-ray flux, is established by their simultaneous
occurrence and similar pulse structure in two independent energy bandpasses.
The light-time distance corresponding to the timescale of their FWHM indicates
that they originate in the inner region of the accretion disk around the
system's black hole component. The fluence in the pulses can equal or exceed
the fluence of the system's average continuous flux over the duration of the
pulses' FWHM in several different bandpasses between 1 and 73 keV. Millisecond
pulses are detected during both high and low luminosity states of Cyg X-1, and
during transitions between luminosity states.
