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

¹ Departament d’Astronomia i Meteorologia Institut de Ciènces del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
e-mail: vmdelacita@am.ub.es
² Department of Physics, Rikkyo University 3-34-1, Nishi-Ikebukuro, Toshima-ku, 171-8501 Tokyo, Japan
³ Departament d’Astronomia i Astrofísica, Universitat de València, Av. Vicent Andrés Estellés s/n, 46100 Burjassot, València, Spain
⁴ Observatori Astronòmic, Universitat de València, C/ Catedràtic José Beltran, 2, 46980 Paterna, València, Spain

Received: 29 July 2015
Accepted: 15 April 2016

Abstract

Context. Stars and their winds can contribute to the non-thermal emission in extragalactic jets. Because of the complexity of jet-star interactions, the properties of the resulting emission are closely linked to those of the emitting flows.

Aims. We simulate the interaction between a stellar wind and a relativistic extragalactic jet and use the hydrodynamic results to compute the non-thermal emission under different conditions.

Methods. We performed relativistic axisymmetric hydrodynamical simulations of a relativistic jet interacting with a supersonic, non-relativistic stellar wind. We computed the corresponding streamlines out of the simulation results and calculated the injection, evolution, and emission of non-thermal particles accelerated in the jet shock, focusing on electrons or e^±-pairs. Several cases were explored, considering different jet-star interaction locations, magnetic fields, and observer lines of sight. The jet luminosity and star properties were fixed, but the results are easily scalable when these parameters are changed.

Results. Individual jet-star interactions produce synchrotron and inverse Compton emission that peaks from X-rays to MeV energies (depending on the magnetic field), and at ~100–1000 GeV (depending on the stellar type), respectively. The radiation spectrum is hard in the scenarios explored here as a result of 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 inverse Compton spectrum because of the Klein-Nishina effect in the cross section. Doppler boosting has a strong effect on the observer luminosity.

Conclusions. 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.