5 Conclusion

From the proper motion and radial velocity of GRO J1655-40 - irrespective of the uncertainties in the distance - we conclude the following:

1.

The X-ray binary has a runaway velocity of 112 $\pm$ 18 km s^-1, probably imparted by a natal explosion during the formation of the black hole.

2.

The linear momentum of the binary system is 538 $\pm$ 100 $M_{\odot}$ km s^-1, which is comparable to that found in solitary neutron stars and millisecond pulsars. This suggests that the relatively low-mass black hole ($\leq$6 $M_{\odot}$) in GRO J1655-40 may have been formed through a neutron star phase.

3.

The kinetic energy of the system is (5.8 $\pm$ 0.4) $\times$ 10⁴⁷ ergs, namely, $\sim$6 $\times$ 10^-4, that of a typical supernova.

4.

The galactocentric orbital plane is almost parallel to the Galactic disk and the Galactic pericentre $\geq$3 kpc, which indicates that the black hole in GRO J1655-40 is the remnant of a massive star born in the Galactic disk.

5.

Contrary to the binary XTE J1118+480 which harbors a black hole born in the halo (Mirabel et al. 2001), in the case of GRO J1655-40 there is unambigous kinematical evidence that the black hole was born in the disk and received a natal kick, most likely from a supernova explosion (Israelian et al. 1999).

Acknowledgements

We thank J. Casares, P. Podsiadlowski, M.E. Beer, J.-P. Lasota, S. Corbel, P. Benaglia, A. Piatti, and E. Gourgoulhon for help and discussions on different aspects of this work. J.A.C. and I.F.M. are members of the Consejo Nacional de Investigaciones Científicas y Técnicas of Argentina, and I.R. is a fellow of the Conselho Nacional de Desenvolvimento Cienífico e Tecnológico of Brazil. I.F.M. acknowledges support from PIP 0049/98 and Fundación Antorchas.