Numerical simulations of super-critical black hole accretion flows in general relativity

A new general relativistic radiation magnetohydrodynamical code koral is described, which employs the M1 scheme to close the radiation moment equations. The code has been successfully verified against a number of tests. Axisymmetric simulations of super-critical magnetized accretion on non-rotating (a_* = 0.0) and spinning (a_* = 0.9) black holes are presented. The accretion rates in the two models are $$\dot{M}\approx 100{\rm -}200 \dot{M}_{\rm Edd}$$. These first general relativistic simulations of super-critical black hole accretion are potentially relevant to tidal disruption events and hyper-accreting supermassive black holes in the early Universe. Both simulated models are optically and geometrically thick, and have funnels through which energy escapes in the form of relativistic gas, Poynting flux and radiative flux. The jet is significantly more powerful in the a_* = 0.9 run. The net energy outflow rate in the two runs correspond to efficiencies of 5 per cent (a_* = 0) and 33 per cent (a_* = 0.9), as measured with respect to the mass accretion rate at the black hole. These efficiencies agree well with those measured in previous simulations of non-radiative geometrically thick discs. Furthermore, in the a_* = 0.9 run, the outflow power appears to originate in the spinning black hole, suggesting that the associated physics is again similar in non-radiative and super-critical accretion flows. While the two simulations are efficient in terms of total energy outflow, both runs are radiatively inefficient. Their luminosities are only ∼1–10L_Edd, which corresponds to a radiative efficiency ∼0.1 per cent. Interestingly, most of the radiative luminosity emerges through the funnels where the local radiative flux is highly super-Eddington.