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The Fate Of Cygnus X-1: An NS-BH Binary Pair?


As one of the brightest X-ray sources in the Milky Way, Cygnus X-1 is regarded as an object of tremendous astronomical interest. Its nature is now well understood to be a black hole of mass 14.8±1.0 (Orosz et al. 2011) coupled to the hot, blue OB associate HDE 226868 (Bolton 1972; Walborn 1973; Gies & Bolton 1982) of mass 19.2±1.9  (Orosz et al. 2011) with an orbital period of 134.4hrs (Seward & Charles 2010). Indeed it was the association with HDE 226868 by Webster & Marian (1972) that conclusively demonstrated that it is, in fact, a binary X-ray emitting system.

Fig. 1: Cygnus X-1 X-ray source and its location in the Cygnus constellation just to the left of η-star. (Credit: Torsten Bronger.)

N.B. The compact object in Cygnus X-1 can be considered a black hole, and not a neutron star, due to its mass. The established upper limit for a neutron star is 2, with a typical neutron star having a masses of 1.35-2.05 ( Lattimer & Prakash 2001; Thorsett et al. 2010), so a compact object of mass ~15 is very likely to be a black hole. Hence, the compact object is well above the theoretical Tolman–Oppenheimer–Volkoff cutoff limit (Celotti et al. 1999)

Recently, the distance to this system was determined by radio parallax methods (see Reid et al. 2011), which opened up new avenues of research on Cyg X-1. Using the parallax distance measures allowed Orosz et al (2011) to firmly establish the binary parameters: an inclination of i = 27.06±0.76° (degrees), an eccentricity of e = 0.018±0.002 and a distance of D = 1.86±0.12kpc (kilo-parsecs).

Understanding the properties of the progenitors of both neutron stars and black holes, as well as considering the constraints on the current mass of HDE 226868 and the compact object by Orosz et al. (2011), paves the way to a greater understanding of the fate of Cygnus X-1. In an attempt to further understand the fate of such high-mass binaries, Bulik, Belczynski & Prestwich (2011) examined specific systems with well established parameters and investigating their future evolution. Although Cygnus X-1 was not a target of this study, the methods outlined can still be effectively utilised in much the same way.

If a binary is chosen close to the end of its life (e.g. before the subsequent formation of double compact object) such a method has potentially great predictive power as many unknowns relating to its prior binary and stellar evolution can be avoided. In particular, Bulik et al. (2011) considered two high mass X-ray binaries (HMXBs), IC10 X-1 and NGC300 X-1, and showed that these systems will soon form close black hole-black hole (BH-BH) systems that will merge within Hubble time and produce strong gravitational radiation (GR).

The end states of Cygnus X-1, specifically its optical OB companion, starts with HDE 226868 swelling in size; eventually overflowing its Roche lobe. The mass which is overflowing will spill out onto the black hole primary, modeled as an advected thin disc (see Abramowicz et al. 1988; Ohsuga et al. 2005). This will, surprisingly, allow the emissions to exceed the classical Eddington limit (Belczynski et al. 2008b). The mass of the black hole will increase to ~17-18, while the optical companion decreases to 4.2, eventually becoming a massive He core with an H-rich outer envelope. Within a few millions years, the star will explode as a hydrogen-poor (Type Ibc) supernova, leaving behind a neutron star.

From this, Belczynski et al. (2011) predict that ~2 of mass loss due to the supernova, a figure not significant to disrupt the Cygnus X-1 system on its own. However, their model suggests that the system may be disrupted because the pre-supernova binary is rather wide with semi-major axis of 1.16A.U. Thus, any significant kick tends to disrupt the binary. As expected for such a wide system, the binary disruption and formation of two single compact objects is most likely: > 74%. Although, albeit less likely but still quite probable, is the formation of a wide BH-NS system: 6-26%. The least likely scenario being the formation of a close BH-NS system with the coalescence time below Hubble time (13.47 Gyr): ~0.5% likely.

So, not quite the exciting result that it could have been. Nonetheless, due to the likelihood predictions, there are dramatic implications for the discovery of BH-NS binaries in the future.

Journal References:

  • De Loore, C.; van Rensbergen, W. (2005) The Evolution Of Massive BinariesAstrophysics & Space Science296, (1-4) pp. 337-352.
  • Belczynski, D.K. et al. (2011) The Fate Of Cyg X-1: An Empircal Limit On BH-NS Merger RateThe Astrophysical Journal737 (2) Article I.D.: 31.
  • Bulik, T. et al. (2011) IC10 X-1/NGC300 X-1: The Very Immediate Progenitors Of BH-BH BinariesThe Astrophysical Journal730 (2) Article I.D.: 140.
  • Orosz, J.A. et al. (2011) Determining The Mass Of The Black Hole In Cygnus X-1. DOI: .2011arXiv1106.3689O.
  • Reid, M.J. et al. (2011) Distance Measurement Using The Trigonometric Parallax Of Cygnus X-1. DOI: 2011arXiv1106.3688R.

Suggested Further Reading:

  • Prialnik, D. (2010) The Theory Of Stellar Structure & Evolution. (2nd Edition) Cambridge University Press, Cambridge.

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