Prompt emission and early optical afterglow of VHE detected GRB 201015A and GRB 201216C: onset of the external forward shock

TL;DR

This paper analyzes the prompt emission and early optical afterglow of two VHE-detected GRBs, revealing insights into their spectral evolution, initial Lorentz factors, and diversity in luminosity, contributing to understanding GRB physics.

Contribution

It provides the earliest optical observations constraining outflow parameters of VHE GRBs and compares their properties with similar bursts, highlighting diversity in luminosity.

Findings

Spectral index remained below synchrotron line of death during burst.

Optical light curves showed a smooth bump indicating afterglow onset.

Constrained initial Lorentz factors to 204 and 310 for the two GRBs.

Abstract

We present a detailed prompt emission and early optical afterglow analysis of the two very high energy (VHE) detected bursts GRB 201015A and GRB 201216C, and their comparison with a subset of similar bursts. Time-resolved spectral analysis of multi-structured GRB 201216C using the Bayesian binning algorithm revealed that during the entire duration of the burst, the low energy spectral index ($\alpha_{\rm pt}$) remained below the limit of the synchrotron line of death. However, statistically some of the bins supported the additional thermal component. Additionally, the evolution of spectral parameters showed that both peak energy (Ep) and $\alpha_{\rm pt}$ tracked the flux. These results were further strengthened using the values of the physical parameters obtained by synchrotron modeling of the data. Our earliest optical observations of both bursts using FRAM-ORM and BOOTES robotic telescopes displayed a smooth bump in their early optical light curves, consistent with the onset of the afterglow due to synchrotron emission from an external forward shock. Using the observed optical peak, we constrained the initial bulk Lorentz factors of GRB 201015A and GRB 201216C to $\Gamma_0$ = 204 and $\Gamma_0$ = 310, respectively. The present early optical observations are the earliest known observations constraining outflow parameters and our analysis indicate that VHE-detected bursts could have a diverse range of observed luminosity within the detectable redshift range of present VHE facilities.