Recollimation shocks and radiative losses in extragalactic relativistic jets

TL;DR

This paper uses advanced simulations to study how radiative losses influence the formation of narrow, focused regions in relativistic jets, explaining rapid gamma-ray variability in quasars.

Contribution

It demonstrates that radiative losses can cause strong jet focusing and stationary nozzles, providing insights into jet dynamics and variability in active galactic nuclei.

Findings

Radiative losses induce strong jet focusing and narrow nozzles.

Simulations reproduce rapid gamma-ray flaring observed in quasars.

Jet structure evolution is sensitive to external medium interactions.

Abstract

We present the results of state-of-the-art simulations of recollimation shocks induced by the interaction of a relativistic jet with an external medium, including the effect of radiative losses of the shocked gas. Our simulations confirm that -- as suggested by earlier semi-analytical models -- the post-shock pressure loss induced by radiative losses may lead to a stationary equilibrium state characterized by a very strong focusing of the flow, with the formation of quite narrow nozzles, with cross-sectional radii as small as $10^{-3}$ times the length scale of the jet. We also study the time-dependent evolution of the jet structure induced of a density perturbation injected at the flow base. The set-up and the results of the simulations are particularly relevant for the interpretation of the observed rapid variability of the $\gamma$-ray emission associated to flat spectrum radio quasars. In particular, the combined effects of jet focusing and Doppler beaming of the observed radiation make it possible to explain the sub-hour flaring events such as that observed in the FSRQ PKS 1222+216 by MAGIC.