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ABSTRACT  Recent observations in X‐rays and gamma‐rays of nearby Fanaroff–Riley type I (FR I) radio galaxies have raised the question of the origin of the emission detected in the termination structures of their jets. The study of these structures can give information on the conditions for particle acceleration and radiation at the front shocks. In addition, an evolutionary scenario can help to disentangle the origin of the detected X‐ray emission in young FR I sources, like some gigahertz peaked spectrum active galactic nuclei. This work focuses on the nature and detectability of the radiation seen from the termination regions of evolving FR I jets. We use the results of a relativistic, two‐dimensional numerical simulation of the propagation of an FR I jet, coupled with a radiation model, to make predictions for the spectra and light curves of the thermal and non‐thermal emission at different stages of the FR I evolution. Our results show that under moderate magnetic fields, the synchrotron radiation would be the dominant non‐thermal channel, appearing extended in radio and more compact in X‐rays, with relatively small flux variations with time. The shocked jet synchrotron emission would dominate the X‐ray band, although the shocked interstellar/intracluster media thermal component alone may be significant in old sources. Inverse Compton (IC) scattering of cosmic microwave background photons could yield significant fluxes in the GeV and TeV bands, with a non‐negligible X‐ray contribution. The IC radiation would present a bigger angular size in X‐rays and GeV than in TeV, with fluxes increasing with time. We conclude that the thermal and non‐thermal broad‐band emission from the termination regions of FR I jets could be detectable for sources located up to distances of a few 100 Mpc.

The evolution of the large‐scale emission in Fanaroff–Riley type I jets