A compact fireball model of gamma-ray bursts

It is proposed that the gamma-ray burst (GRB) photons near the peak of the spectrum at several hundred keV are produced on very compact scales, where photon production is limited by blackbody effects and/or the requirement of energetic quanta (E > 2m_e c²) for efficient further production. The fast variation of order milliseconds in the time profile is then a natural expectation, given the other observed GRB parameters. Analytic calculations are presented to show that the escape of nonthermal, energetic gamma rays can emerge within 1 s of the thermal photons from a gammasphere of below 10¹² cm. The minimum asymptotic bulk Lorentz factor in this model is found to be of order several hundred if the photosphere is of order 3 × 10¹¹ cm and greater for larger or smaller photospheric radii. It is suggested that prompt ultra-high-energy gamma rays might provide a new constraint on the asymptotic Lorentz factor of the outflow.