The Wolf-Rayet Black Hole Binary IC 10 X-1
Wolf-Rayet stars are some of the most massive stars known (Beals 1933), typically ranging from 25 to 75 (and perhaps more!), which loose mass at an incredible rate: up to one billion times that of our tiny Sun every year (Crowther 2007). As their are only some 300 Wolf-Rayet stars known to date (van der Hucht 2001, 2006) finding these stars as members of high mass X-ray binaries is something of a bind. Although, our understanding of stellar evolution predicts that they should exist (Van den Heuvel & De Loore 1973).
High mass X-ray binaries (HMXB) typically comprise OB stars with either a neutron star or black hole as its compact stellar companion (Crowther et al. 2009). If a merger is avoided, the OB companion will potentially evolve through to the Wolf-Rayet phase, perhaps producing a system comprising a Helium (He) star with its black hole or neutron star companion (Tutukov & Yungelson 1973).
Recent theories on HMXBs outline the plausible existence of massive He stars with a compact object companion (e.g. Ergma & Yungelson 1998). Such systems are believed to be extremely rare; however, Ergma & Yungelson (1998) have outlined a possible haul of ~100 such HMXB systems could be found in our Galaxy; with only a limited amount from this conservative estimate being visible as X-ray sources. To date, there are only three galactic and extra-galactic candidates: NGC 300 X-1 (Crowther et al. 2010), Cygnus X-3 (van den Heuvel & de Loore 1973; Lommen et al. 2005) and IC 10 X-1 (Silverman & Fillipenko 2008).
IC 10 X-1 is a highly luminous X-ray source which can be found in the local group starburst dwarf irregular galaxy ‘IC 10’ (Bauer & Brandt 2004). Confirming the collapsar model for compact stellar objects, IC 10 X-1 is surrounded by shells of X-ray emission (Wang et al. 2005); thus, confirming that there must have been a supernova explosion in that system at some point, thus associating itself with the compact remnant in IC 10 X-1.
Although there are three likely candiates for Wolf-Rayet black hole binary (WR-BHB) systems, IC 10 X-1 was the first to be discovered (Crowther et al. 2003). However, I must make it clear that there are at least 4 plausible counterparts to the compact X-ray source, although the most probable high mass companion is the Wolf-Rayet star MAC92 17-A. Coupled with its extraordinary discovery as the first WR-BHB observed, the system has a period of 34.9hrs eluding to the fat that the unseen compact companion exceeds 23.1±2.1 (Prestwhich et al. 2007): the largest of all the known stellar sized black holes!
The fact that one of the largest stellar sized black holes is coupled to a massive Wolf-Rayet star comes as no surprise. Stars form in huge clouds of gas and dust (Shu et al. 1987). If a number of these clouds contain more gases, then it would be a reasonable assumption to state that the stars born here would become more massive. Hence, we have massive stars forming in a relatively close region of space, especially so in starburst galaxies like IC 10 (Leitherer et al. 1999). Hence, if one of these stars is born slightly more massive than it’s companion, it will evolve faster: becoming the black hole associated with IC 10 X-1, whilst it’s lower mass associated straggles behind as a Wolf-Rayet star.
However, one slight problem remains with our IC 10 X-1 system. High black hole masses require that the progenitor star was very massive and experienced low mass-loss rates (Belczynski et al. 2009). Yet, Crowther et al. (2009) outlined that due to the orbital period of IC10 X–1, the radius of the black hole progenitor star must have been larger than the present separation of the components. As such, the progenitor would have experienced extreme mass-loss through Roche-lobe overflow. Therefore, reconciling high black hole masses with close orbital separations is a major challenge for current binary evolution models.
- Leitherer, C.A et al. (1999) Synthesis Models For Starburst Galaxies With Active Star Formation. The Astrophysical Journal Supplement Series, 123 (1) pp. 3-40.
- Prestwhich, A.H. et al. (2007) The Orbital Period Of The Wolf-Rayet Binary IC 10 X-1: Dynamic Evidence That The Compact Object Is A Black Hole. DOI: arXiv:0709.2892v1.
- Crowther, P.A. (2008) Properties Of Wolf-Rayet Stars. International Astronomical Union Proceedings: IAU Symposium, 250 pp. 47-62.
- Crowther, P.A. et al. (2010) NGC 300 X-1 Is A Wolf-Rayet Black Hole Binary. Monthly Notices Royal Astronomical Society: Letters, 403 (1) pp.L41-L45.
- Belczynski, D.K. et al. (2011) IC10 X-1/NGC300 X-1: The Very Immediate Progenitors Of BH-BH Binaries. The Astrophysical Journal, 730 (2) Article I.D.: 140.