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

TL;DR

This study compares gamma-ray burst afterglows with blazar sequences, finding similarities with high frequency-peaked BL Lac objects and suggesting common emission processes, while also noting deviations due to optical extinction in dark bursts.

Contribution

It demonstrates that GRB afterglows follow blazar spectral sequences and are similar to high frequency-peaked BL Lac objects, revealing shared emission mechanisms.

Findings

GRB afterglows align with the blazar sequence, especially high frequency-peaked BL Lacs.

Dark bursts deviate from the sequence, indicating significant optical extinction.

Optical extinction in dark bursts is estimated to be greater than 0.5-0.6 magnitudes.

Abstract

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.