Understanding compact object formation and natal kicks. III. the case of Cygnus X-1

Tsing Wai Wong*, Francesca Valsecchi, Tassos Fragos, Vassiliki Kalogera

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

70 Scopus citations


In recent years, accurate observational constraints have become available for an increasing number of Galactic X-ray binaries (XRBs). Together with proper-motion measurements, we could reconstruct the full evolutionary history of XRBs 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 all available observational constraints. Our analysis accounts for three evolutionary phases: orbital evolution and motion through the Galactic potential after the formation of a 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, and at the time of core collapse, the BH has potentially received a small kick velocity of ≤77kms-1 at 95% confidence. If the BH progenitor mass is less than ∼17 M , 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 does not 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 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.

Original languageEnglish (US)
Article number111
JournalAstrophysical Journal
Issue number2
StatePublished - Mar 10 2012


  • X-rays: binaries
  • X-rays: individual (Cygnus X-1)
  • binaries: close

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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