Neutron stars are known to have large transverse motions on the plane of the sky which are believed to result from natal kicks imparted by supernova explosions. Energetic explosions have also been invoked in models of the core collapse of massive stars onto black holes. However, there have been few observations that constrain models of the physical processes by which stellar-mass black holes are formed. The measurement of a large radial velocity for the centre of mass of the black hole X-ray binary GRO J1655-40 (Orosz & Bailyn 1997; Shahbaz et al. 1999), together with chemical elements found by Israelian et al. (1999) on the surface of the donor star, provided observational support to the idea that black holes - as neutron stars- may form in supernova explosions that impart strong natal kicks to the collapsed objects.
If a black hole is accompanied by a mass-donor star, it is possible to determine the radial velocity, proper motion, and distance of the system, from which one can derive the space velocity, track the path to the site of birth, and constrain the strength of the natal kick. Presently, the most accurate proper motions of X-ray binaries are obtained following at radio wavelengths with Very Long Baseline Interferometry (VLBI) the motion in the sky of the associated compact microquasar jets, as done recently for the halo black hole binary XTE J1118+480 (Mirabel et al. 2001). This has not been possible for GRO J1655-40 because there is no VLBI calibrator nearby, and in recent years the radio counterpart faded away below the detection limit. In this context, we carried out relative astrometry of the secondary star using optical images obtained with the Hubble Space Telescope 6.3 years apart. Here we report the proper motion of GRO J1655-40 which, together with the radial velocity, allows us - taking into account the uncertainties in the distance - to determine the parameters of its runaway kinematics and galactocentric orbit.
Copyright ESO 2002