2011ApJ...726L...4W

The spectral properties from radio-to-optical bands are compared between the 18 optically bright gamma-ray burst (GRB) afterglows and the well-established power-spectrum sequence in blazars. The comparison shows that the afterglows are in well agreement with the well-known blazar sequence (i.e., the νL _ν(5 GHz)-α_ROcorrelation, where α_RO is the broadband spectral slope from radio-to-optical bands). The afterglows are, however, clustered at the low-luminosity end of the sequence, which is typically occupied by high-frequency-peaked BL Lac objects (HBLs). The correlation suggests that GRB afterglows share a similar emission process with HBLs. We further identify a deviation at a significance level larger than 2σ from the sequence for three typical optically "dark" bursts. The deviation favors a heavy extinction in optical bands for the "dark" bursts. The extinction A_V is estimated to be larger than 0.5-0.6 mag from the νL_ν(5 GHz)-α_RO sequence.