BODO, GianluigiGianluigiBODOTAVECCHIO, FabrizioFabrizioTAVECCHIO2020-11-092020-11-0920180004-6361http://hdl.handle.net/20.500.12386/28224We 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<SUP>-3</SUP> 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.STAMPAenArticle10.1051/0004-6361/2017320002-s2.0-85041395638000429204300002https://www.aanda.org/articles/aa/abs/2018/01/aa32000-17/aa32000-17.html2018A&A...609A.122BFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_10 High energy and particle astronomy – X-rays, cosmic rays, gamma rays, neutrinos