  We present Chandra X-ray monitoring of the M87 jet in 2002, which shows that
the intensity of HST-1, an optical knot 0.8" from the core, increased by a
factor of two in 116 days and a factor of four in 2 yrs. There was also a
significant flux decrease over two months, with suggestive evidence for a
softening of the spectrum. From this variability behavior, we argue that the
bulk of the X-ray emission of HST-1 comes from synchrotron emission. None of
the other conceivable emission processes can match the range of observed
characteristics. By estimating synchrotron model parameters for various bulk
relativistic velocities, we demonstrate that a model with a Doppler factor,
delta, in the range 2 to 5 fits our preliminary estimates of light travel time
and synchrotron loss timescales.
