We report on X-ray observations of the large-scale jets recently discovered in the radio and detected in X-rays from the black hole candidate X-ray transient and microquasar XTE J1550-564. On 2002 March 11, X-ray emission was detected 23'' to the west of the black hole candidate and was extended along the jet axis with an FWHM of 12 and a full width at 10% of maximum intensity of 5''. The morphology of the X-ray emission matched well to that of the radio emission at the same epoch. The jet moved by 052 ± 013 between 2002 March 11 and June 19. The apparent speed during that interval was 5.2 ± 1.3 mas day-1. This is significantly less than the average apparent speed of 18.1 ± 0.4 mas day-1 from 1998 to 2002, assuming that the jet was ejected in 1998 September, and indicates that the jet has decelerated. The X-ray spectrum is adequately described by a power law with a photon index near 1.8 subject to interstellar absorption. The unabsorbed X-ray flux was 3.4 × 10-13 ergs cm-2 s-1 in the 0.3-8 keV band in 2002 March and decreased to 2.9 × 10-13 ergs cm-2 s-1 in June. We also detect X-rays from the eastern jet in 2002 March and show that it has decelerated and dimmed since the previous detections in 2000.
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