Analytical modelling of AGN outflows

Abstract

We study AGN jet formation by means of exact solutions of the relativistic mag- netohydrodynamic equations. This is an original extension in Schwarzchild's metric of previous meridionally self-similar models. The outflow is mainly thermally driven to high asymptotic speeds from the central region where the escape speed is also high. The jet solutions are mag- netically collimated on the sub-parsec scale. Almost all AGNs are associated with outflows. These phenomena of high energy affect the evolution of the central object and its surrounding medium. In fact, observations show that jets are launched very close to the black hole where the thermal and mag- netic energies are sufficiently high such that the plasma escapes from the gravitational potential. Many analytical and numerical works have been developed to examine the mechanisms of acceleration and collimation of jets, few of them though in the frame of general relativity. Jet formation seems to be strictly related to the presence of large-scale magnetic fields and the existence of gaseous disks (Konigl & Pudritz 2000). The origin can be the Keplerian disk or the spherical corona surrounding the central part of the AGN or the microquasar as discussed by several authors. All MHD models sustain the idea that the gravitational energy of the central object is transferred to the accreted matter and eventually the jet. This energy is carried by the flow along the magnetic field lines anchored in the corona of the rotating disk. The hot ionized fluid close to the axis where the rotation is weak may escape from the corona thermally in the form of a wind and forced to follow the field lines. Then rotation combined to the magnetic field colli- mates the flow beyond the Alfvsurface. In this contribution we present a model for such thermally driven and magnetically confined outflows from a spherical hot corona. The terminal velocity is, as expected, of the order of the escape speed at the footpoints.