Pair loading in gamma-ray burst fireballs and prompt emission from pair-rich reverse shocks

Gamma-ray bursts (GRBs) are believed to originate from ultrarelativistic winds/fireballs to avoid the "compactness problem." However, the most energetic photons in GRBs may still suffer from gamma-gamma absorption leading to e(+/-) pair production in the winds/fireballs. We show here that in a wide range of model parameters, the resulting pairs may dominate those electrons associated with baryons. Later on, the pairs would be carried into a reverse shock so that a shocked pair-rich fireball may produce a strong flash at lower frequencies, i.e., in the IR band, in contrast to optical/UV emission from a pair-poor fireball. The IR emission would show a 5/2 spectral index due to strong self-absorption. Rapid responses to GRB triggers in the IR band would detect such strong flashes. The future detections of many IR flashes will infer that the rarity of prompt optical/UV emissions is in fact due to dust obscuration in the star formation regions.