General relativistic magnetohydrodynamical κ-jet models for Sagittarius A

Context. The observed spectral energy distribution of an accreting supermassive black hole typically forms a power-law spectrum in the near infrared (NIR) and optical wavelengths, that may be interpreted as a signature of accelerated electrons along the jet. However, the details of acceleration remain uncertain. Aims. In this paper, we study the radiative properties of jets produced in axisymmetric general relativistic magnetohydrodynamics (GRMHD) simulations of hot accretion flows onto underluminous supermassive black holes both numerically and semi-analytically, with the aim of investigating the differences between models with and without accelerated electrons inside the jet. Methods. We assume that electrons are accelerated in the jet regions of our GRMHD simulation. To model them, we modify the electrons' distribution function in the jet regions from a purely relativistic thermal distribution to a combination of a relativistic thermal distribution and the kappa-distribution function (the kappa-distribution function is itself a combination of a relativistic thermal and a non-thermal power-law distribution, and thus it describes accelerated electrons). Inside the disk, we assume a thermal distribution for the electrons. In order to resolve the particle acceleration regions in the GRMHD simulations, we use a coordinate grid that is optimized for modeling jets. We calculate jet spectra and synchrotron maps by using the ray tracing code RAPTOR, and compare the synthetic observations to observations of Sgr A*. Finally, we compare numerical models of jets to semi-analytical ones. Results. We find that in the kappa-jet models, the radio-emitting region size, radio flux, and spectral index in NIR/optical bands increase for decreasing values of the kappa parameter, which corresponds to a larger amount of accelerated electrons. This is in agreement with analytical predictions. In our models, the size of the emission region depends roughly linearly on the observed wavelength lambda, independently of the assumed distribution function. The model with kappa = 3.5, eta(acc) = 5-10% (the percentage of electrons that are accelerated), and observing angle i = 30 degrees fits the observed Sgr A* emission in the flaring state from the radio to the NIR/optical regimes, while kappa = 3 : 5, eta(acc) < 1%, and observing angle i = 30 degrees fit the upper limits in quiescence. At this point, our models (including the purely thermal ones) cannot reproduce the observed source sizes accurately, which is probably due to the assumption of axisymmetry in our GRMHD simulations. The kappa-jet models naturally recover the observed nearly-flat radio spectrum of Sgr A* without invoking the somewhat artificial isothermal jet model that was suggested earlier. Conclusions. From our model fits we conclude that between 5% and 10% of the electrons inside the jet of Sgr A* are accelerated into a kappa distribution function when Sgr A* is flaring. In quiescence, we match the NIR upper limits when this percentage is < 1%.