Electron–positron pair production in global GRMHD simulations of black hole accretion flows

ABSTRACT We present global, 3D general-relativistic magnetohydrodynamic simulations of accreting black holes that incorporate pair physics. Pairs are modelled as a passive scalar that maintains a constant temperature. For high accretion rate models, we observe a maximum pair fraction of $\sim \mathcal {O}(0.01)$, consistent with those inferred from some X-ray binaries, and identify a ‘pair void’ extending to a few gravitational radii from the black hole. Pair fractions peak in the mid-plane just outside the plunging region and within a thin strip at the base of the corona. For moderate to high accretion rate models, pairs are near equilibrium close to the disc mid-plane, where the scattering optical depth is high and pair equilibrium time-scales are short, and could be comparable to the Coulomb collision time-scale. This suggests the possibility of a pair-regulated coronal temperature. In contrast, the upper corona and jets, where the scattering optical depth is relatively low and pair equilibrium time-scales are long, are populated with pairs that may exceed their equilibrium value by orders of magnitude. These pairs are transported by advection from the disc, which dominates over local pair processes. This result highlights advection as a significant source of pair injection, which may be relevant for certain X-ray binaries exhibiting $\gamma$-ray signatures. The pair density along the magnetically dominated poles exceeds the Goldreich–Julian density in some models.