  The source of emission from Sgr A*, the supermassive black hole at the
Galactic Center, is still unknown. Flares and data from multiwavelength
campaigns provide important clues about the nature of Sgr A* itself. Here we
attempt to constrain the physical origin of the broadband emission and the
radio flares from Sgr A*. We developed a time-dependent jet model, which for
the first time allows one to compare the model predictions with flare data from
Sgr A*. Taking into account relevant cooling mechanisms, we calculate the
frequency-dependent time lags and photosphere size expected in the jet model.
The predicted lags and sizes are then compared with recent observations. Both
the observed time lags and size-frequency relationships are reproduced well by
the model. The combined timing and structural information strongly constrain
the speed of the outflow to be mildly relativistic, and the radio flares are
likely to be caused by a transient increase in the matter channelled into the
jets. The model also predicts light curves and structural information at other
wavelengths which could be tested by observations in the near future. We show
that a time-dependent relativistic jet model can successfully reproduce: (1)
the quiescent broadband spectral energy distribution of Sgr A*, (2) the
observed 22 and 43 GHz light curve morphologies and time lags, and (3) the
frequency-size relationship. The results suggest that the observed emission at
radio frequencies from Sgr A* is most easily explained by a stratified,
optically thick, mildly relativistic jet outflow. Frequency-dependent
measurements of time-lags and intrinsic source size provide strong constraints
on the bulk motion of the jet plasma.
