Jet-lagged: Revealing the Nature of Sgr A* through Timing and VLBI

Is the radio emission of the Galaxy's supermassive black hole candidate Sgr A* coming from an accretion flow or a jet-like outflow? The recent discovery of internal radio structure in Sgr A* through mm-VLBI observations now allows constraining the hitherto hotly disputed nature of this emission, at scales from a few out to some hundred Schwarzschild radii. The frequency-dependent size implies a stratified, optically thick plasma with the highest frequencies corresponding to the smallest scales. Moreover, measurements of the radio variability show a clear time lag between 22 GHz and 43 GHz such that the higher frequencies, i.e. smaller scales, lead the lower frequencies, i.e. larger scales. Since the source is optically thick, time lag and structural information together provide an indication of the actual bulk flow velocity and direction. At 43 GHz this data then implies an outflow from the black hole close to the speed of light, rather than an inflow. An outflow at this speed would be unbound. Velocity and structure are quite consistent with the long-standing predictions of the jet model and difficult to reconcile with accretion models. Future timing observations, together with the VLBI results, will therefore offer the exciting prospects for mapping the velocity field of the radio emitting plasma in great detail. This will allow for the first time to observe the collimation and acceleration of an astrophysical jet close to its nozzle. Relativistic beaming effects from fast rotation and outflow will also modify the expected appearance of the black hole shadow in the innermost 5 Schwarzschild radii, e.g. producing arclets, thus allowing smaller structures to be seen at submm-wavelength with future VLBI experiments.