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

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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 multistructured GRB 201216C using the Bayesian binning algorithm revealed that during the entire duration of the burst, the low-energy spectral index (αpt) 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 the peak energy (Ep) and αpt 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 the F/Photometric Robotic Atmospheric Monitor Observatorio del Roque de los Muchachos and Burst Observer and Optical Transient Exploring System 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 Γ0 = 204 and Γ0 = 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. © 2023. The Author(s). Published by the American Astronomical Society.
R.G. and S.B.P. acknowledge the financial support of ISRO under AstroSat archival Data utilization program (DS_2B-13013(2)/1/2021-Sec.2). A.A. acknowledges funds and assistance provided by the Council of Scientific & Industrial Research (CSIR), India with file No. 09/948(0003)/2020-EMR-I. A.J.C.T. acknowledges support from the Spanish Ministry project PID2020-118491GB-I00 and Junta de Andalucia grant P20_010168. Y.D.H. acknowledges support under the additional funding from the RYC2019-026465-I. M.C.G. acknowledges support from the Ramón y Cajal Fellowship RYC2019-026465-I (funded by the MCIN/AEI/10.13039/501100011033 and the European Social Funding). This research has used data obtained through the HEASARC Online Service, provided by the NASA-GSFC, in support of the NASA High Energy Astrophysics Programs. S.B.P. thankfully acknowledges inclusion of the photometric calibration data of GRB 201015A taken with the 4K × 4K CCD Imager acquired as a part of the present analysis and extends sincere thanks to the observing and support staff of the 3.6 m DOT for maintaining and running the observational facilities at Devasthal Nainital.
With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).