Coupling hydrodynamics and radiation calculations for star-jet interactions in active galactic nuclei

Abstract

Stars and their winds can contribute to the non-thermal (NT) emission in extragalactic jets. Given the complexity of jet-star interactions, the properties of the resulting emission are strongly linked to those of the emitting flows. We simulate the interaction between a stellar wind and a relativistic extragalactic jet and use the hydrodynamic results to compute the NT emission under different conditions. We perform relativistic axisymmetric hydrodynamical simulations of a relativistic jet interacting with a supersonic, non-relativistic stellar wind. We compute the corresponding streamlines out of the simulation results, and calculate the injection, evolution, and emission of NT particles accelerated in the jet shock, focusing on electrons or $e^\pm$-pairs. Several cases are explored, considering different jet-star interaction locations, magnetic fields and observer lines of sight. The jet luminosity and star properties are fixed, but the results are easily scalable under changes of these parameters. Individual jet-star interactions produce synchrotron and inverse Compton (IC) emission that peaks from X-rays to MeV energies (depending on the magnetic field), and at $\sim 100-1000$ GeV (depending on the stellar type), respectively. The radiation spectrum is hard in the scenarios explored here due to non-radiative cooling dominance, as low-energy electrons are efficiently advected even under relatively high magnetic fields. Interactions of jets with cold stars lead to an even harder IC spectrum because of the Klein-Nishina effect in the cross-section. Doppler boosting has a strong impact on the observer luminosity. The emission levels for individual interactions found here are in the line of previous, more approximate, estimates, strengthening the hypothesis that collective jet-star interactions could significantly contribute at high energies under efficient particle acceleration.