  We present the analysis of the Chandra X-ray Observatory observations of the
eccentric gamma-ray binary PSR B1259-63/LS 2883. The analysis shows that the
extended X-ray feature seen in previous observations is still moving away from
the binary with an average projected velocity of about 0.07c and shows a hint
of acceleration. The spectrum of the feature appears to be hard (photon index
of 0.8) with no sign of softening compared to previously measured values. We
interpret it as a clump of plasma ejected from the binary through the
interaction of the pulsar with the decretion disk of the O-star around
periastron passage. We suggest that the clump is moving in the unshocked
relativistic pulsar wind (PW), which can accelerate the clump. Its X-ray
emission can be interpreted as synchrotron radiation of the PW shocked by the
collision with the clump.
