  We have performed a series of Chandra X-ray Observatory observations of the
gamma-ray binary LS 2883, which is comprised of a young pulsar (PSR B1259-63)
orbiting a massive Be star with a period of 1236.7 days. The system was
observed in 5 epochs, spanning a range from 352 to 1175 days after the
periastron passage on 2014 May 4. The observations confirmed the recurrent
nature of the high-speed ejecta that appear as an extended X-ray structure
(clump) moving away from the binary. Compared to the results of the previous
monitoring campaign (between the 2010 and 2014 periastron passages), this time
we find evidence suggesting that the clump is accelerated to a projected
velocity $ v_{\perp}\approx0.15c$ with an acceleration $a_{\perp}=47\pm2$ cm
s$^{-2}$ (for uniformly accelerated motion), assuming that it was launched near
periastron passage. The observed X-ray properties of the clump are consistent
with synchrotron emission from pulsar wind particles accelerated at the
interface between the pulsar wind and the clump. We have also performed
contemporaneous observations with the {\sl Hubble Space Telescope}, which are
used to set an upper limit on the optical flux of the extended emission.

High Energy Astrophysical Phenomena