Annu. Rev. Astron. Astrophys. 1999. 37: 409-443
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3.3. Superluminal Motions in XTE J1748-288

Two major relativistic ejection sequences moving at least 20 mas day-1 were observed in June 1998 (Hjellming et al 1998) from thehard X-ray transient XTE J1748-288 (Smith et al 1998). Each sequence appeared to begin with a one-sided relativistic ejection. The ejecta are highly linearly polarized, and at a distance of 8 kpc, derived from the HI lambda21cm absorption line, their motions would imply apparent speeds of 0.9c and 1.5c, and intrinsic velocities of more than 0.9c (Hjellming et al 1998). This is the first galactic source of relativistic jets where it has been observed in real time that the jets collide with environmental material, being decelerated while brightening at the leading edge of the jet.