Unveiling black holes ejected from globular clusters

Was the black hole in XTE J1118+480 ejected from a globular cluster or kicked away from the galactic disk? 1. The intriguing origin of the kinematic of XTE J1118 + 480 Strong dynamical evidence now exists for 17 black holes (BHs) in X-ray novae but only a few have been observed away from the galactic plane (White & van Paradijs 1996). XTE J1118 + 480 is a high galactic latitude (b = 62.3) BH X-ray nova having an exceptionally large mass function f = 6.0 ± 0.4 M⊙ and the shortest orbital period (P=4.08 h) among the BH binaries. The donor star is probably a main sequence star of ∼ 0.3 M⊙. The transverse motion on the plane of the sky indicates that the system is now orbiting with a space velocity of ∼ 145 km s relative to the Local Standard of Rest (Mirabel et al 2001), much higher than the velocity dispersion of the galactic BH X-ray binaries (40 km s). The most natural birthplace of the BH in XTE J1118 + 480 would be the Disk of the Galaxy, from which a kick could have propelled XTE J1118 + 480 into the halo. This invokes two alternatives. Disk-A: An asymmetric natal kick as large (∼ 200 km/s) as never recorded in the case of a black hole. Given the scaling Vkick,BH = Vkick,NS(mNS/mBH) α where α < 1 (Colpi & Wasserman 2002), such kick is compatible with the high velocity end of the observed distribution of the transverse velocities of the pulsars (Arzoumanian, Chernoff & Cordes 2002). Disk-B: A symmetric kick from recoil due to supernova explosion (typically of order ∼ 40 km/s), superimposed to the contribution of the circular velocity (> 100 km/s) of the galactic disk at the site of the supernova explosion. In Figure 1 we show the orbit followed by XTE J1118 + 480 during the last 3 Gyrs; we have verified that some of the transits across the galactic disk are cinematically consistent with the hypothesis Disk-B. A completely different scenario calls for XTE J1118+480 to be ejected from a globular cluster (GC). Two expulsion mechanisms have been explored. GC-A: A three-body exchange interaction (where a single BH intrudes into a main-sequence star binary, MS+ms, replacing the less massive star, ms) occurred in the cluster core in the first stages of its evolution. However, given the recoil energy of ∼ 10 erg needed to eject the BH+MS binary from a GC of typical escape speed 30-50 km s, the initial MS+ms binary should have been extremely tight (at most 3 − 4 R⊙) for accomplishing both momentum and energetic re-