An inhomogeneous lepto-hadronic model for the radiation of relativistic jets. Application to XTE J1118+480

Context. Conceptually reconstructing the physical conditions in relativistic jets, given the observed electromagnetic spectrum, is a complex inverse problem. Aims. We aim to improve our understanding of the mechanisms operating in relativistic jets by modeling their broadband electromagnetic spectrum. Methods. We develop an inhomogeneous jet model where the injection of relativistic primary and secondary particles takes place in a spatially extended region. We calculate the contribution of all particles species to the jet emissivity by several radiative processes, and assess the effect of gamma-ray absorption in internal and external photon fields. A number of specific models with different parameters are computed to explore the possibilities of this scenario. Results. We obtain a variety of spectral shapes depending on the model parameters, some of them predicting significant gamma-ray emission. The observed broadband spectrum of the low-mass microquasar XTE J1118+480 can be satisfactorily reproduced by the model. Conclusions. Our results indicate that outbursts similar to those displayed in the past by XTE J1118+480 might be detected with present-day gamma-ray instruments.