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Discerning the radiative dissipation mechanism for prompt emission inGamma-Ray Bursts (GRBs) requires detailed spectroscopic modeling that straddlesthe $\nu F_{\nu}$ peak in the 100 keV - 1 MeV range. Historically, empiricalfits such as the popular Band function have been employed with considerablesuccess in interpreting the observations. While extrapolations of the Bandparameters can provide some physical insight into the emission mechanismsresponsible for GRBs, these inferences do not provide a unique way ofdiscerning between models. By fitting physical models directly this degeneracycan be broken, eliminating the need for empirical functions; our analysis hereoffers a first step in this direction. One of the oldest, and leading,theoretical ideas for the production of the prompt signal is the synchrotronshock model (SSM). Here we explore the applicability of this model to a bright{\it Fermi} GBM burst with a simple temporal structure, GRB {\it 090820}A. Ourinvestigation implements, for the first time, thermal and non-thermalsynchrotron emissivities in the RMFIT forward-folding spectral analysissoftware often used in GBM burst studies. We find that these synchrotronemissivities, together with a blackbody shape, provide at least as good a matchwith the data as the Band GRB spectral fitting function. This success isachieved in both time-integrated and time-resolved spectral fits.

Constraints on the Synchrotron Shock Model for the Fermi GBM Gamma-Ray Burst 090820A