Hipparcos results on massive X-ray binaries

Abstract

We present results on parallaxes, magnitudes and proper motions for 17 optically identified massive X-ray binaries (MXRB) which were observed during the Hipparcos astrometry mission. This sample includes the sources detected in the medium-energy range (2-10 keV) having optical counterparts brighter than V=12. We compare the Hipparcos results with ground-based optical data and derive probable values for absolute magnitudes and peculiar tangential velocities (v_t). The 4 OB supergiant systems in our sample are high-velocity objects (the average value of v_t is greater than 60 km/s), while the 13 Be systems have low velocities (<v_t> = 11.3 +/- 6.7 km/s), suggesting a different formation mechanism for the two subgroups. The unusual X-ray/radio source LSI+61 303 lies much closer than previously believed and its low optical luminosity (M_V = +2.2) and blue intrinsic colors now suggest it may be a peculiar type of low-mass X-ray binary.