Horizon-Scale Lepton Acceleration in Jets: Explaining the Compact Radio Emission in M87

TL;DR

This paper proposes that pair formation and particle acceleration at the stagnation surface near black holes can explain the compact radio emission in M87's jet, aligning well with observations and suggesting a link to gamma-ray luminosity in blazars.

Contribution

It introduces a new mechanism involving inverse-Compton pair catastrophe at the stagnation surface as the source of relativistic leptons in jets, providing quantitative agreement with observations.

Findings

The mechanism explains the relativistic lepton densities in M87's jet.

It accounts for the absence of a jet in Sagittarius A*.

It predicts a relationship between jet power and gamma-ray luminosity.

Abstract

It has now become clear that the radio jet in the giant elliptical galaxy M87 must turn on very close to the black hole. This implies the efficient acceleration of leptons within the jet at scales much smaller than feasible by the typical dissipative events usually invoked to explain jet synchrotron emission. Here we show that the stagnation surface, the separatrix between material that falls back into the black hole and material that is accelerated outward forming the jet, is a natural site of pair formation and particle acceleration. This occurs via an inverse-Compton pair catastrophe driven by unscreened electric fields within the charge-starved region about the stagnation surface and substantially amplified by a post-gap cascade. For typical estimates of the jet properties in M87, we find excellent quantitive agreement between the predicted relativistic lepton densities and those required by recent high-frequency radio observations of M87. This mechanism fails to adequately fill a putative jet from Sagittarius A* with relativistic leptons, which may explain the lack of an obvious radio jet in the Galactic center. Finally, this process implies a relationship between the kinetic jet power and the gamma-ray luminosity of blazars, produced during the post-gap cascade.