6752 hiding a double black hole binary in its core ?

NGC6752 hosts in its halo PSR J1911-5958A, a newly discovered binary millisecond pulsar which is the most distant pulsar ever known from the core of a globular cluster. Interestingly, its recycling history seems in conflict with a scenario of ejection resulting from ordinary stellar dynamical encounters. A scattering event off a binary system of two black holes with masses in the range of 3 − 50M⊙ that propelled PSR J1911-5958A into its current peripheral orbit seems more likely. It is still an observational challenge to unveil the imprint(s) left from such a dark massive binary on cluster’s stars: PSR J1911-5958A may be the first case. 1. Binary black holes in globular clusters It is known that a significant number of black holes (BHs) of stellar origin should form in the first million years in globular clusters (GCs), with masses > 3M⊙. These BHs segregate toward the GC cores by dynamical friction where they form double binaries [BH+BH] and multiple systems. With time, BH−[BH+BH] exchange interactions become overwhelmingly important and most of the BHs are ejected by recoil (Sigurdsson & Hernquist 1993). The last [BH+BH] binary(ies) however will have no other BHs to eject and will remain in the cluster’s core (Portegies Zwart & McMillan 2000). In this cosmic dance some of the interacting BHs can grow in mass up to ∼ 100M⊙ by cannibalizing other BHs (Miller & Hamilton 2002) and these will be preferentially retained in the GC due to their higher inertia (a high mass BH could also be primordial, born from the heaviest stars). Figure 1 shows, for a binary with masses [mBH + MBH ∼ 50M⊙], the two lower bounds on the separation a, denoted with arec and aGW, as a function of mBH, below which recoil by an incoming BH of 10 M⊙ (or of mBH), and gravitational wave (GW) emission would eject the binary and cause coalescence, respectively. Can such a massive dark [BH+BH] binary in a GC be detected? GCs host a number of millisecond pulsars (MSPs) tracing the population of neutron stars (NSs) recycled to short periods in accretion events, after capturing the mass-losing star in dynamical encounters. We here show that PSR J19115958A, a canonical binary MSP, can probe the existence of a [BH+BH] binary in NGC6752.