1999ApJ...520L..25H

Numerous reports have been made of features, in either emission or absorption, in the 10-1000 keV spectra of some gamma-ray bursts. Originally interpreted in the context of Galactic neutron star models as cyclotron line emission and e⁺-e^– annihilation features, the recent demonstration that the majority of gamma-ray bursts (GRBs) lie at cosmological distances makes these explanations unlikely. In this Letter, we adopt a relativistic fireball model for cosmological GRBs in which dense, metal-rich blobs or filaments of plasma are entrained in the relativistic outflow. In the context of this model, we investigate the conditions under which broadband features, similar to those detected, can be observed. We find a limited region of parameter space capable of reproducing the observed GRB spectra. Finally, we discuss possible constraints additional high-energy spectral observations could place on fireball model parameters.