1998A&A...331L..29E

We examine the information on the progenitor masses of stellar black holes that can be derived from the seven known black hole X-ray binaries with low- mass donor stars (Soft X-ray Transients- SXRs) in combination with stellar evolution considerations and observed properties of High Mass X-ray Binaries (HMXBs). It appears that several independent lines of evidence indicate that a considerable fraction of the stars more massive than 20-25M_☉ leave black holes as remnants while, on the other hand, t e neutron star in at least one HMXB had a progenitor more massive than 50M_☉. We argue that the only plausible explanation of these apparently contradicting facts is that in a certain mass range, probably between 20 and 50M_☉, the outcome of core collapse can either be a neutron star or the black hole, depending on additional stellar parameters, for example rotation and magnetic fields. The high masses of the black holes in three soft X-ray Transients (6-13M_☉) and Cygnus X-1 (16M_☉) exclude the alternative explanation that in a limited mass range (e.g. 20-40M_☉) stars leave black holes, and, due to very heavy stellar wind mass loss, stars more massive than about 40M_☉ leave neutron stars, just as stars between 8 and 20M_☉.