Black Hole X-ray Transients: The Formation Puzzle

There are 19 confirmed BH binaries in the Galaxy. 16 of them are X-ray transients hosting a ~5-15 Msun BH and a Roche-lobe overflowing low-mass companion. Companion masses are found mostly in 0.1-1 Msun mass range with peak at 0.6 Msun. The formation of these systems is believed to involve a common envelope phase, initiated by a BH progenitor, expected to be a massive star >20 Msun. It was realized that it may be very problematic for a low-mass companion to eject a massive envelope of the black hole progenitor. It invoked suggestions that an intermediate-mass companion ejects the envelope, and then is shredded by the Roche-lobe overflow to its current low-mass. But this creates another issue; a temperature mismatch between hot models and the observed cool low-mass donors. Finally, the main driver of Roche-lobe overflow that is believed to be magnetic braking does not seem to follow any theoretically calculated models. Number of ideas were put forward to explain various parts of this conundrum; pre-main sequence donor nature, alternative approach to magnetic braking and common envelope energy was revisited. We test various proposals and models to show that no overall solution exists so far. We argue that common envelope physics is not crucial in the understanding BH transient physical properties, but may affect significantly their formation rates. Our failure most likely indicates that either the current evolutionary models for low-mass stars and magnetic braking are not realistic or that the intrinsic population of BH transients is quite different from the observed one.