Swift identification of dark gamma-ray bursts

We present an optical flux vs. X-ray flux diagram for all known gamma-ray bursts (GRBs) for which an X-ray afterglow has been detected. We propose an operational definition of dark bursts as those bursts that are optically subluminous with respect to the fireball model, i.e., which have an optical-to-X-ray spectral indexOX < 0.5. Out of a sample of 52 GRBs we identify 5 dark bursts. The definition and diagram serve as a simple and quick diagnostic tool for identifying dark GRBs based on limited information, particularly useful for early and objective identification of dark GRBs observed with the Swift satellite. Subject headings: dust, extinction — galaxies: high-redshift — gamma rays: bursts large differences in localisation accuracies, localisation time since the onset of the burst, and search strate- gies. Moreover, effects of observing conditions (e.g., lu- nar phase) have generally not been taken into account in statistical studies. In many cases, GRBs have been con- sidered dark if no OA was detected, irrespective of how inefficient the search was. In fact, there is no generally accepted criterion for when a GRB is considered dark. With the launch of the Swift satellite it will be essential to have a quick diagnostic tool to flag dark bursts for im- mediate and/or detailed follow-up (including the near-IR bands) to ensure homogeneity of samples. In this Let- ter we present a GRB diagram of the optical flux (Fopt) vs. the X-ray flux (FX) and propose that those bursts which are optically subluminous with respect to the fire- ball model, i.e., which have an optical-to-X-ray spectral indexOX < 0.5, be defined as dark.