Snapshot Identification of Gamma-Ray Burst Optical Afterglows

Gamma-ray burst afterglows can be identified in single-epoch observations using three or more optical filters. This method relies on color measurements to distinguish the power-law spectrum of an afterglow from the curved spectra of stars. Observations in a fourth filter will further distinguish between afterglows and most galaxies up to redshifts z~1. Many afterglows can also be identified with fewer filters using ultraviolet-excess, infrared-excess, or Lyman break techniques. By allowing faster identification of gamma-ray burst afterglows, these color methods will increase the fraction of bursts for which optical spectroscopy and other narrow-field observations can be obtained. Because quasar colors can match those of afterglows, the maximum error box size where an unambiguous identification can be expected is set by the flux limit of the afterglow search and the quasar number-flux relation. For currently typical error boxes (10-100 arcmin²), little contamination is expected at magnitudes R<~21.5+/-0.5. Archival data demonstrate that the afterglow of GRB 000301C could have been identified using this method. In addition to finding gamma-ray burst counterparts, this method will have applications in ``orphan afterglow'' searches used to constrain gamma-ray burst collimation. Dedicated to the memory of Jan van Paradijs.