Survey of radiative, two-temperature magnetically arrested simulations of the black hole M87* I: turbulent electron heating

ABSTRACT We present a set of 11 two-temperature, radiative, general relativistic magnetohydrodynamic simulations of the black hole M87* in the magnetically arrested state, surveying different values of the black hole spin $a_*$. Our 3-D simulations self-consistently evolve the temperatures of separate electron and ion populations under the effects of adiabatic compression/expansion, viscous heating, Coulomb coupling, and synchrotron, bremsstrahlung, and inverse Compton radiation. We adopt a subgrid heating prescription from gyrokinetic simulations of plasma turbulence. Our simulations have accretion rates $\dot{M}=(0.5-1.5)\times 10^{-6}\dot{M}_{\rm Edd}$ and radiative efficiencies $\epsilon _{\rm rad}=$ 3–35 per cent. We compare our simulations to a fiducial set of otherwise identical single-fluid general relativistic magnetohydrodynamic (GRMHD) simulations and find no significant changes in the outflow efficiency or black hole spin-down parameter. Our simulations produce an effective adiabatic index for the two-temperature plasma of $\Gamma _{\rm gas}\approx 1.55$, larger than the $\Gamma _{\rm gas}=13/9$ value often adopted in single-fluid GRMHD simulations. We find moderate ion-to-electron temperature ratios in the 230 GHz emitting region of $R=T_{\rm i}/T_{\rm e}\,{\approx }\,5$. While total intensity 230 GHz images from our simulations are consistent with Event Horizon Telescope (EHT) results, our images have significantly more beam-scale linear polarization ($\langle |m|\rangle \approx 30~{{\rm per\ cent}}$) than is observed in EHT images of M87* ($\langle |m|\rangle \lt 10~{{\rm per\ cent}}$). We find a trend of the average linear polarization pitch angle $\angle \beta _2$ with black hole spin consistent with what is seen in single-fluid GRMHD simulations, and we provide a simple fitting function for $\angle \beta _2(a_*)$ motivated by the wind-up of magnetic field lines by black hole spin in the Blandford–Znajek mechanism.