Gamma-ray burst spectral features: Interpretation as X-ray emission from aphotoionized plasma

Numerous reports have been made of features, in either emission or absorpti on, in the 10-1000 keV spectre of some gamma-ray bursts. Originally interpr eted in the context of Galactic neutron star models as cyclotron line emiss ion and e(+)-e(-) annihilation features, the recent demonstration that the majority of gamma-ray bursts (GRBs) lie at cosmological distances makes the se explanations unlikely. In this Letter, we adopt a relativistic fireball model for cosmological GRBs in which dense, metal-rich blobs or filaments o f plasma are entrained in the relativistic outflow. In the context of this model, we investigate the conditions under which broadband features, simila r 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 pl ace on fireball model parameters.