The light curve and spectrum of the abnormally luminous type Ic SN 1998bw (Galama et al. Galama et al. (1998)) suggest that in this event a black hole was formed (Iwamoto et al. Iwamoto et al. (1998)). The observations imply that a considerable fraction of the mass of the progenitor (a massive C/O core) was ejected in the explosion (Iwamoto et al. Iwamoto et al. (1998)). Similarly, the observed overabundance of the elements O, Mg, Si and S in the atmosphere of the companion of Nova Sco 1994, indicates considerable mass ejection in the formation of this black hole Israelian et al. 1999.
From a study of the z-distribution of the population of black hole X-ray binaries with low mass donors, White & van Paradijs White and van Paradijs (1996) conclude that the velocity dispersion of these X-ray binaries is of the order of 40 km s-1. Since the velocity dispersion of the progenitor systems is expected to be around 17 km s-1, they estimate the extra velocity that is given to the system in the formation of the black hole to be 20 - 40 km s-1. This requires substantial mass ejection in the formation of a black hole if no asymmetric kicks are involved.
Recent determinations of the space velocity of Cyg X-1 Kaper et al. 1999 and the radial velocity of Nova Sco Bailyn et al. 1995 demonstrate that these black hole binaries have significantly higher runaway velocities than the black hole X-ray binaries with low mass donors.
In Table 1 we have listed the relevant properties of the galactic black hole binaries for which one, or more, of its velocity components have been measured.
Table 1. Properties of black hole X-ray binaries. The velocity of Cyg X-1 is its space velocity, all other velocities are radial velocities, so are lower limits Notes: Masses and periods from Ergma & van den Heuvel Ergma and van den Heuvel (1998) and references therein, velocities from Brandt et al. Brandt et al. (1995) and references therein, except for GRO J0422+32 Harlaftis et al. 1999 and GS2000+25 Harlaftis et al. 1996 which are heliocentric -velocities. For Nova Sco 1994 we changed the velocity according to the new -velocity of 142 km s-1 Shahbaz et al. 1999. Space velocity for Cyg X-1 is from Kaper et al. Kaper et al. (1999).
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999