Tsing-Wai Wong, Francesca Valsecchi, Tassos Fragos, Vassiliki Kalogera
In recent years, accurate observational constraints become available for an
increasing number of Galactic X-ray binaries. Together with proper motion
measurements, we could reconstruct the full evolutionary history of X-ray
binaries back to the time of compact object formation. In this paper, we
present the first study of the persistent X-ray source Cygnus\;X-1 that takes
into account of all available observational constraints. Our analysis accounts
for three evolutionary phases: orbital evolution and motion through the
Galactic potential after the formation of black hole (BH), and binary orbital
dynamics at the time of core collapse. We find that the mass of the BH
immediate progenitor is $15.0 - 20.0$ M$_\sun$, and at the time of core
collapse, the BH has potentially received a small kick velocity of $\le 77$ km
s$^{-1}$ at 95% confidence. If the BH progenitor mass is less than $\sim 17$
M$_\sun$, a non zero natal kick velocity is required to explain the currently
observed properties of Cygnus\;X-1. Since the BH has only accreted mass from
its companion's stellar wind, the negligible amount of accreted mass is
impossible to explain the observationally inferred BH spin of $a_* > 0.95$, and
the origin of this extreme BH spin must be connected to the BH formation
itself. Right after the BH formation, we find that the BH companion is a $19.8
- 22.6$ M$_\sun$ main sequence star, orbiting the BH at a period of $4.7 - 5.2$
days. Furthermore, recent observations show that the BH companion is currently
super-synchronized. This super-synchronism indicates that the strength of tides
exerted on the BH companion should be weaker by a factor of at least two
compared to the usually adopted strength.
View original:
http://arxiv.org/abs/1107.5585
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