Recollimation shocks and radiative losses in extragalactic relativistic jets

¹ INAF–Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: bodo@to.astro.it
² INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy

Received: 26 September 2017
Accepted: 17 October 2017

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 by 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 γ-ray emission associated to flat spectrum radio quasars. In particular, the combined effects of jet focusing and Doppler beaming on the observed radiation make it possible to explain the sub-hour flaring events such as that observed in the flat specrum radio quasar PKS 1222+216 by MAGIC.