Dynamics of extended emission associated with the high-mass gamma-ray binary B1259-63

We report the results of monitoring of the eccentric B1259-63 binary with the Chandra X-ray Observatory following the 2014 periastron passage. The observations confirm that the X-ray emitting clumps are ejected from the binary each binary cycle in the same (periastron-apastron) direction. 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 being accelerated to a projected velocity of about 0.15c on a timescale of hundreds of days. Assuming that the X-ray clump is launched near preceding periastron passage, the measured acceleration (~50 cm/s^2) sets an upper limit on the mass and kinetic energy of the clump. 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 clump. We discuss the possible connection with the gamma-ray flares observed by Fermi LAT and expectations for the next binary orbital cycle.