Compton rockets and the minimum power of relativistic jets

TL;DR

This paper investigates the minimum power of relativistic jets in gamma-ray sources, emphasizing the role of leptons and protons, and introduces the Compton rocket effect as a key factor in jet dynamics.

Contribution

It presents a novel analysis of jet power constraints considering radiative cooling, jet composition, and the Compton rocket effect, providing lower limits based on observational data.

Findings

Minimum jet power exceeds accretion luminosity for typical viewing angles.

The jet composition likely includes no more than about 10 pairs per proton.

The Compton rocket effect influences jet deceleration and power estimates.

Abstract

The power of a relativistic jet depends on the number of leptons and protons carried by the jet itself. We have reasons to believe that powerful gamma-ray flat spectrum radio sources emit most of their radiation where radiative cooling is severe. This helps to find the minimum number of emitting leptons needed to explain the radiation we see. The number of protons is more uncertain. If there is one proton per electron, they dominate the jet power, but they could be unimportant if the emission is due to electron-positron pairs. In this case the total jet power could be much smaller. However, if the gamma-ray flux is due to inverse Compton scattering with seed photons produced outside the jet, the radiation is anisotropic also in the comoving frame, making the jet to recoil. This Compton rocket effect is strong for light, electron-positron jets, and negligible for heavy, proton dominated jets. No significant deceleration, required by fast superluminal motion, requires a minimum number of protons per lepton, and thus a minimum jet power. We apply these ideas to the blazar 3C 454.3, to find a robust lower limit to its total jet power: if the viewing angle theta_v ~ 1/Gamma the jet power is larger than the accretion luminosity L_d for any bulk Lorentz factor Gamma. For theta_v =0, instead, the minimum jet power can be smaller than L_d for Gamma<25. No more than ~10 pairs per proton are allowed.