Black hole X-ray transients: The formation puzzle
Abstract
There are 19 confirmed black hole binaries in the Galaxy, 16 of which are X-ray transients hosting a ~5-15 Msun black hole and a Roche-lobe-overflowing low-mass companion. Companion masses are found mostly in 0.1-1 Msun mass range, with a peak at 0.6 Msun. The formation of these systems is believed to involve a common envelope (CE) phase, initiated by a black hole progenitor, expected to be a rather massive star >20 Msun. We show that none of the available CE models allow for the formation of the observed population of Galactic black hole transients. Black holes are most likely found with companions of mass 1 Msun for the standard CE energy prescription. The companion mass distribution also peaks at 1 Msun and is still in tension with observations, even if the envelope binding energy is lowered by factor of 5 as suggested by Ivanova & Chaichenets (2011). The angular momentum CE prescription produces a rather flat companion mass distribution over a broad range of 0.5-1.3 Msun with significantly fewer systems than observed in the Galaxy. Additionally, we have modified the most important factors shaping the companion mass distribution: magnetic braking, the mass-transfer rate, and the star-formation rate. In each case, almost no companions are found around the observed mass peak. We argue that CE physics is not crucial for our understanding of Galactic BH transients. 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 black hole transients is quite different from that observed.
- Publication:
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Binary Systems, their Evolution and Environments
- Pub Date:
- September 2014
- DOI:
- 10.48550/arXiv.1312.5924
- arXiv:
- arXiv:1312.5924
- Bibcode:
- 2014bsee.confE..37W
- Keywords:
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- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Submitted to ApJ