Photometric and Spectroscopic Observations of GRB 190106A: Emission from Reverse and Forward Shocks with Late-time Energy Injection

TL;DR

This paper reports early optical and spectroscopic observations of GRB 190106A, revealing emission from reverse and forward shocks, and provides insights into the central engine and energy injection processes during the afterglow phase.

Contribution

First optical observations of GRB 190106A during prompt emission, confirming the reverse-forward shock model with late-time energy injection.

Findings

Double-peak optical light curve consistent with reverse-forward shock model

Redshift measured as z=1.861 from spectroscopic data

Optical and X-ray light curves show shallow then normal decay

Abstract

Early optical observations of gamma-ray bursts can significantly contribute to the study of the central engine and physical processes therein. However, of the thousands observed so far, still only a few have data at optical wavelengths in the first minutes after the onset of the prompt emission. Here we report on GRB\,190106A, whose afterglow was observed in optical bands just 36 s after the {\em Swift}/BAT trigger, i.e., during the prompt emission phase. The early optical afterglow exhibits a bimodal structure followed by a normal decay, with a faster decay after $\sim \rm T_{0}+$1 day. We present optical photometric and spectroscopic observations of GRB\,190106A. We derive the redshift via metal absorption lines from Xinglong 2.16-m/BFOSC spectroscopic observations. From the BFOSC spectrum, we measure $z= 1.861\pm0.002$. The double-peak optical light curve is a significant feature predicted by the reverse-forward external shock model. The shallow decay followed by a normal decay in both the X-ray and optical light curves is well explained with the standard forward-shock model with late-time energy injection. Therefore, GRB\,190106A offers a case study for GRBs emission from both reverse and forward shocks.