GRB 191016A: The onset of the forward shock and evidence of late energy injection

Abstract

We present optical and near-infrared photometric observations of GRB 191016 with the COATLI, DDOTI and RATIR groundbased telescopes over the first three nights. We present the temporal evolution of the optical afterglow and describe 5 different stages that were not completely characterized in previous works, mainly due to scarcity of data points to accurately fit the different components of the optical emission. After the end of the prompt gamma-ray emission, we observed the afterglow rise slowly in the optical and near-infrared (NIR) wavelengths and peak at around 𝑇 + 1450 s in all filters. This was followed by an early decay, a clear plateau from 𝑇 + 5000 s to 𝑇 + 11000 s, and then a regular late decay. We also present evidence of the jet break at later times, with a temporal index in good agreement with the temporal slope obtained from X-ray observations. Although many of the features observed in the optical light curves of GRBs are usually well explained by a reverse shock (RS) or forward shock (FS), the shallowness of the optical rise and enhanced peak emission in the GRB191016A afterglow is not well-fitted by only a FS or a RS. We propose a theoretical model which considers both of these components and combines an evolving FS with a later embedded RS and a subsequent late energy injection from the central engine activity. We use this model to successfully explain the temporal evolution of the light curves and discuss its implications on the fireball properties.

Onset of particle acceleration during the prompt phase in gamma-ray bursts as revealed by synchrotron emission in GRB160821A

“…Finally, the ratio of pulse-width to pulse-time for the main episode 40 s/135 s < 1, which indicates that it is due to late central engine activity (Lazzati & Perna 2007;Pereyra et al 2021) causing an external-shock flare. This is reminiscent of strong gamma-ray flares observed at the end of the prompt phase (Zhang et al 2018) and X-ray flares observed after the prompt activity in the gamma-ray band (Hu et al 2014), which all require a long-lived central engine activity.…”

Onset of Particle Acceleration during the Prompt Phase in Gamma-Ray Bursts as Revealed by Synchrotron Emission in GRB 160821A

“…This fact is supported by the quiescent period observed just before the 130 s flare in GRB 160821A. Finally, the ratio of pulse width to pulse time for the main episode is 40 s/135 s < 1, which indicates late central engine activity (Lazzati & Perna 2007;Pereyra et al 2021) causes an external-shock flare. This is reminiscent of strong gamma-ray flares observed at the end of the prompt phase (Zhang et al 2018) and X-ray flares observed after the prompt activity in the gamma-ray band (Hu et al 2014), which all require a longlived central engine activity.…”