4 Prediction of new fireball types

In Dermer et al. (1999a), we found that the behavior of the temporally-evolving emission is robust to orders-of-magnitude changes in all parameter values except for q and $\Gamma_0$. The effect of changing q is to translate $P(\epsilon,t)$ horizontally along the $\epsilon$ axis by the factor q. The value of q can be constrained, at least during the prompt phase of the GRB, by spectral modeling.

The observability of a fireball is most profoundly affected by the value of $\Gamma_0$. For clean fireballs with small baryon loading ($\Gamma_0\gg 300$), we find that GRBs are intense, subsecond, medium-to-high energy ($\gg 10$ MeV) $\gamma$-ray events, and are difficult to detect because of inadequate photon counts given the insufficiently large effective areas ($\sim10^3$ cm²) of > 100 MeV $\gamma$-ray detectors such as EGRET on CGRO and deadtime limitations. Dirty fireballs ($\Gamma_0\ll 300$) produce transient emissions which are longer lasting and most luminous at X-ray energies and below, but these events are lost behind the glow of the X-ray and lower-energy background radiations except for rare serendipitous detections by pointed instruments.