Pair multiplicity around a supermassive black hole with superalfvénic jets

In the vicinity of a supermassive black hole, magnetohydrodynamics (MHD) simulations and models of hot accretion flow predicted a density of injected pairs divided by the Goldreich–Julian density – the pair multiplicity – in the range 1–103. When the reconnection mechanism in the magnetosphere is introduced, the multiplicity could reach 108–109 and can explain the observed superluminal radio blobs. Here we show that the range 1–103 is also difficult to reconcile with observations at the Alfvén point of the M87 jet on the polar axis at few tens of gravitational radii. An axisymmetric and frozen flux MHD approach allows to show that the pair multiplicity on the Alfvén point only depends on the black hole mass, the fluid temperature and velocity, the magnetic field, and the isorotation frequency. Normalizing our calculations from observational constraints on M87 jet and with reasonable assumptions, we obtain pair multiplicity around 109. In conclusion, for a pure pair plasma, the velocity and magnetic field deduced from observations of superalfvénic jets lead to high values of the pair multiplicity at the Alfvén point.