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.
