Models for "optically dark" gamma-ray bursts

Roughly 65% percent of long gamma-ray bursts (GRBs) are "optically dark"; i.e., no optical afterglow is detected for these bursts. Three explanations of these "optically dark" GRBs have been widely discussed: (1) the optical afterglow is extinguished by dust in the vicinity of the GRB or in the star-forming region in which the GRB occurs; (2) some GRBs have optical afterglows that are intrinsically very faint; and (3) the GRB lies at very high redshift (z > 5), and the optical afterglow is absorbed by neutral hydrogen in the host galaxy and in the intergalactic medium along the line of sight from the burst to us. I briefly review and discuss each of these explanations.