Gamma-ray burst afterglows as analogs of high-frequency-peaked BL Lac objects

Abstract

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 νLν(5GHz)-αRO correlation, where αRO 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 AV is estimated to be larger than 0.5-0.6 mag from the νLν(5 GHz)-αRO sequence. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.