How Special Are Dark Gamma‐Ray Bursts: A Diagnostic Tool

We present here a comprehensive study of the optical/near-infrared (IR) upperlimits for gamma-ray bursts that have an X-ray afterglow. We have extrapolatedthe X-ray afterglows to optical wavelengths based on the physics of thefireball blast wave model (e.g. Rees & Meszaors 1992; Meszaros & Rees 1997),and compared these results with optical upper limits for a large sample ofbursts. We find a small set of only three bursts out of a sample of 20 forwhich the upper limits are not compatible with their X-ray afterglow propertieswithin the context of any blast wave model. This sparse sample does not allowus to conclusively determine the cause of this optical/near-IR deficit.Extinction in the host galaxy is a likely cause, but high redshifts anddifferent afterglow mechanisms might also explain the deficit in some cases. Wenote that the three bursts appear to have higher than average gamma-ray peakfluxes. In a magnitude versus time diagram the bursts are separated from themajority of bursts with a detected optical/near-IR afterglow. However, two GRBswith an optical afterglow (one of which is highly reddened) also fall in thisregion with dark bursts, making it likely that dark bursts are at the faint endof the set of optically detected bursts, and therefore the dark bursts likelyform a continuum with the bursts with a detected optical afterglow. Our workprovides a useful diagnostic tool for follow-up observations for potentiallydark bursts; applied to the events detected with the Swift satellite, it willsignificantly increase our sample of truly dark bursts and shed light upontheir nature.Comment: 19 pages; accepted to Ap