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The black hole at the Galactic Center, Sgr A*, is the prototype of a galacticnucleus at a very low level of activity. Its radio through submm-wave emissionis known to come from a region close to the event horizon, however, the sourceof the emission is still under debate. A successful theory explaining theemission is based on a relativistic jet model scaled down from powerfulquasars. We want to test the predictive power of this established jet modelagainst newly available measurements of wavelength-dependent time lags and thesize-wavelength structure in Sgr A*. Using all available closure amplitude VLBIdata from different groups, we again derived the intrinsic wavelength-dependentsize of Sgr A*. This allowed us to calculate the expected frequency-dependenttime lags of radio flares, assuming a range of in- and outflow velocities.Moreover, we calculated the time lags expected in the previously publishedpressure-driven jet model. The predicted lags are then compared to radiomonitoring observations at 22, 43, and 350 GHz. The combination of time lagsand size measurements imply a mildly relativistic outflow with bulk outflowspeeds of gamma*beta ~ 0.5-2. The newly measured time lags are reproduced wellby the jet model without any major fine tuning. The results further strengthenthe case for the cm-to-mm wave radio emission in Sgr A* as coming from a mildlyrelativistic jet-like outflow. The combination of radio time lag and VLBIclosure amplitude measurements is a powerful new tool for assessing the flowspeed and direction in Sgr A*. Future VLBI and time lag measurements over arange of wavelengths will reveal more information about Sgr A*, such as theexistence of a jet nozzle, and measure the detailed velocity structure of arelativistic jet near its launching point for the first time.

Jet-lag in Sgr A*: What size and timing measurements tell us about the central black hole in the Milky Way