Black Hole X-ray Transients: The Formation Puzzle

Understand

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.