RHESSI Tests of Quasi-Thermal Gamma-Ray Burst Spectral Models

Prompt gamma-ray burst (GRB) spectra evolve on short timescales, suggesting that time-resolved spectral fits may help diagnose the still unknown prompt emission mechanism. We use broadband gamma-ray data from the Ramaty High-Energy Solar Spectroscopic Imager spacecraft to test quasi-thermal models with high signal-to-noise time-resolved spectra of nine bright GRBs. In contrast to results reported in more narrow energy bands, the quality of the fits of quasi-thermal models is poor in relation to fits of the phenomenological Band function. Moreover, the best-fit parameters for the simplest quasi-thermal model, a blackbody plus a nonthermal power law, show significant dependence on the fit band. Models that replace the power law with more complicated nonthermal functions are not robust for the data considered here and decrease the physical relevance of the fit blackbody.