Item

Abstract

The detection of rapidly variable TeV emission on timescales of ~ 1 day in M87 implies a source size of a few Schwarzschild radii rs, and suggests that the TeV emission may originate from unscreened electric fields in the black hole magnetospheres. The presence of the VLBA jet, on the other hand, indicates that a force-free (ideal) MHD flow is established on scales < 50 rs. We show that inverse Compton scattering of ambient photons by electrons accelerated in the gap can initiate a pair cascade above the gap, that results in a sufficiently large multiplicity to ensure a force-free flow. Photons having energies below a few TeV can escape freely. The VHE power radiated by the gap can easily account for the luminosity of the TeV source detected by, e.g., H.E.S.S. Intermittencies of the cascade process will give rise to the observed variability of the TeV emission, and the resultant force-free outflow, as indicated by the morphological changes of the VLBA jet.