  Sagittarius A$^*$ in the Galactic center harbors a supermassive black hole
and exhibits various active phenomena. Besides quiescent emission in radio and
submillimeter radiation, flares in the near infrared (NIR) and X-ray bands are
observed to occur frequently. We study a time-dependent model of the flares,
assuming that the emission is from a blob ejected from the central object.
Electrons obeying a power law with the exponential cutoff are assumed to be
injected in the blob for a limited time interval. The flare data of 2007 April
4 were used to determine the values of model parameters. The spectral energy
distribution of flare emission is explained by nonthermal synchrotron radiation
in the NIR and X-ray bands. The model light curves suggest that electron
acceleration is still underway during the rising phase of the flares. GeV
gamma-rays are also emitted by synchrotron self-Compton scattering, although
its luminosity is not strictly constrained by the current model. If the GeV
emission is faint, the plasma blob is dominated by the magnetic energy density
over the electron kinetic energy density. Observations in the GeV band will
clarify the origin of the blob.
