On the initial progenitor masses of stellar mass black holes and neutron stars

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 Xray Binaries (HMXBs). It appears that several independent lines of evidence indicate that a considerable fraction of the stars more massive than 20 - 25 M-. 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 50 M-.. We argue that the only plausible explanation of these apparently contradicting facts is that in a certain mass range, probably between 20 and 50 M-., 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 -13 M-.) and Cygnus X-1 (16 M-.) exclude the alternative explanation that in a limited mass range (e.g.20 - 40 M) stars leave black holes, and, due to very heavy stellar wind mass loss, stars more massive than about 40 M-. leave neutron stars, just as stars between 8 and 20 M-..