Mergers of accreting stellar-mass black holes

We present post-Newtonian N -body simulations on mergers of accreting stellarmass black holes (BHs), where such general relativistic effects as the pericentre shift and gravitational wave emission are taken into consideration. To elucidate the key physics that regulates mergers of BHs, the dynamical friction and the mass accretion by ambient gas are incorporated. We consider a system composed of ten black holes with initial mass of 30 M⊙. As a result, we show that mergers of accreting stellar-mass BHs are classified into four types: a gas drag-driven, an interplay-driven, a three body-driven, or an accretion-driven merger. We find that BH mergers proceed before significant mass accretion, even if the accretion rate is ∼ 10 Eddington accretion rate, and then all BHs can merge into one heavy BH. More specifically, using the simulation results for a wide range of parameters, we derive a critical accretion rate (ṁc), below which the BH growth is promoted faster by mergers: ṁc/ṁHL = 6 × 10 −3 for low-density ambient gas of ngas . 10 cm, and ṁc/ṁHL = 2 × 10 (ngas/10 cm)(ρBH/10 M⊙pc ) for ngas & 10 cm, where ṁHL is the Hoyle-Lyttleton mass accretion rate. Very recently, a gravitational wave event, GW150914, as a result of the merger of a ∼ 30 M⊙ BH binary has been detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) (Abbott et al. 2016). Based on the present simulations, the BH merger in GW150914 is likely to be driven by three-body encounters accompanied by a few M⊙ of gas accretion.