  X-ray emission and absorption features are of great importance in our
understanding the nature and environment of gamma-ray bursts (GRBs). So far,
iron emission lines have been detected in at least four GRB afterglows. In this
paper, the observational properties and physical constraints on materials
surrounding GRB sources are reviewed, and several classes of theoretical models
are also discussed. We will specially concentrate on the Cerenkov line
mechanism, in which the broad iron lines are expected, and a small mass of Fe
is required to produce the large line luminosity. In addition, our
interpretation can favor the recent jet unified model for different classes of
gamma-ray bursts with a standard energy reservoir.
