Gamma-Ray Burst Afterglows as Analogues of High Frequency-Peaked BL Lac Objects

The spectral properties from radio to optical bands are compared between the 18 optically bright Gamma-ray burst afterglows and well established power-spectrum sequence in Blazars. The comparison shows that the afterglows are well agreement with the well known Blazar sequence (i.e., the $\nu L_{\nu}(\mathrm{5GHz})$-$\alpha_{\mathrm{RO}}$ correlation, where $\alpha_{\mathrm{RO}}$ is the broad-band 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. The correlation suggests that Gamma-ray burst afterglows share the similar emission process with high frequency-peaked BL Lac objects. We further identify a deviation at a significance level larger than 2$\sigma$ 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 magnitude from the $\nu L_{\nu}(\mathrm{5GHz})$-$\alpha_{\mathrm{RO}}$ sequence.