Abstract

We propose a new method for the classification of optically dark gamma‐ray bursts (GRBs), based on the X‐ray and optical‐to‐X‐ray spectral indices of GRB afterglows, and utilizing the spectral capabilities of Swift. This method depends less on model assumptions than previous methods, and can be used as a quick diagnostic tool to identify optically sub‐luminous bursts. With this method we can also find GRBs that are extremely bright at optical wavelengths. We show that the previously suggested correlation between the optical darkness and the X‐ray/gamma‐ray brightness is merely an observational selection effect.