A high-velocity black hole on a Galactic-halo orbit in the solar neighbourhood

Only a few of the dozen or so known stellar-mass black holes have been obse rved away from the plane of the Galaxy(1). Those few could have been ejecte d from the plane as a result of a 'kick' received during a supernova explos ion, or they could be remnants of the population of massive stars formed in the early stages of evolution of the Galaxy. Determining their orbital mot ion should help to distinguish between these options. Here we report the tr ansverse motion (in the plane of the sky) for the black-hole X-ray nova XTE J1118+480 (refs 2-5), from which we derive a large space velocity. This X- ray binary system has an eccentric orbit around the Galactic Centre, like m ost objects in the halo of the Galaxy, such as ancient stars and globular c lusters. The properties of the system suggest that its age is comparable to or greater than the age of the Galactic disk. Only an extraordinary 'kick' from a supernova could have launched the black hole into an orbit like thi s from a birthplace in the disk of the Galaxy.