Kinematics of black hole X-ray binary GRS 1915+105

The space velocity of a stellar black hole encodes the history of its formation and evolution. Here we measure the three-dimensional motion of the microquasar GRS 1915+105, using a decade of astrometry with the NRAO Very Long Baseline Array, together with the published radial velocity. The velocity in the Galactic plane deviates from circular rotation by 53-80 +/- 8 km s(-1), where the range covers any specific distance from 6-12 kpc. Perpendicular to the plane, the velocity is only 10 +/- 4 km s(-1). The peculiar velocity is minimized at a distance 9-10 kpc and is then nearly in the radial direction toward the Galactic center. We discuss mechanisms for the origin of the peculiar velocity, and conclude that it is most likely a consequence of Galactic velocity diffusion on this old binary, rather than the result of a supernova kick during the formation of the 14M. black hole. Finally, a brief comparison is made with four other black hole (BH) binaries whose kinematics are well determined.