Constraints on mass ejection in black hole formation derived from black hole X-ray binaries

Both the recently observed high runaway velocities of Cyg X-1 (∼ 50 km s) and X-ray Nova Sco 1994 (≥ 100 km s) and the relatively low radial velocities of the black hole X-ray binaries with low mass donor stars, can be explained by symmetric mass ejection in the supernovae (SNe) which formed the black holes in these systems. Assuming symmetric mass ejection in black hole formation, we estimate the amount of mass that must have been ejected in the stellar core collapse in order to explain the velocities of the above X-ray binaries. We find that at least 2.6M⊙ and 4.1M⊙ must have been ejected in the formation of Cyg X-1 and Nova Sco, respectively. A similar mass loss fraction (f = 0.35 ) for the black hole binaries with low mass donors, gives low velocities, in agreement with the observations. We conclude that the black holes in X-ray binaries are all consistent with being formed in a successful SN in which mass is ejected. A possible kick at the formation of the black hole is not needed to explain their space velocities.