  We have discovered an X-ray jet due to material ejected from the black hole
X-ray transient XTE J1550-564. The discovery was first reported by Corbel et
al. (Science, 298, 196 and astro-ph/0210224), and here, we present an analysis
of the three Chandra observations made between 2000 June and 2000 September.
For these observations, a source is present that moves in an eastward direction
away from the point source associated with the compact object. The separation
between the new source and the compact object changes from 21.3 arcseconds in
June to 23.4 arcseconds in September, implying a proper motion of 21.2 +/- 7.2
mas/day, a projected separation of 0.31-0.85 pc and an apparent jet velocity
between 0.34 +/- 0.12 and 0.93 +/- 0.32 times the speed of light for a source
distance range of d = 2.8-7.6 kpc. These observations represent the first time
that an X-ray jet proper motion measurement has been obtained for any accretion
powered Galactic or extra-galactic source. While this work deals with the jet
to the east of the compact object, the western jet has also been detected in
the X-ray and radio bands. The most likely scenario is that the eastern jet is
the approaching jet and that the jet material was ejected from the black hole
in 1998. Along with a 1998 VLBI proper motion measurement, the Chandra proper
motion indicates that the eastern jet decelerated between 1998 and 2000. We
present results on the morphology and energy spectrum of the jet. We cannot
definitively determine the X-ray emission mechanism, but a synchrotron origin
is viable and may provide the simplest explanation for the observations.
