Observational lower mass limit for black hole formation derived from massive X-ray binaries

Abstract

The high radial velocity amplitude (235 km s−1) and 1.70-day orbital period of the B3V optical counterpart of the X-ray source LMC X-3 set a lower limit of 6–10 M⊙ for the mass of the X-ray source, strongly suggesting that it is a black hole1,2. An observational upper limit to the mass of this compact star can be derived which, in combination with considerations about the evolutionary history of the system, allows us to derive an upper limit to the (zero age) mass of the progenitor of the compact star in this system: ⩽ 80±10 M⊙. From the orbital parameters of pulsating massive X-ray binaries we find that stars with masses up to ⩽40 (±5) M⊙ (and, if supernova (SN) mass ejection is symmetric, up to 60 (±7) M⊙) terminate life as neutron stars. Thus, if LMC X-3 is a black hole, the lower mass limit for terminating life as a black hole is in the range between 40 (60 M⊙) and 80 M⊙. Together with the initial mass function of massive stars this yields a black hole formation rate in the galaxy about two orders of magnitude smaller than the neutron-star formation rate. The direct progenitors of the black holes (just before the supernova) were massive Wolf–Rayet (WR) stars.