| Author(s) and Co-Author(s) with Affiliation: Tanima Mondal(Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, 721302, India), Soebur Razzaque(Centre for Astro-Particle Physics (CAPP) and Department of Physics, University of Johannesburg, PO Box 524, Auckland Park, 2006,), Jagdish C. Joshi(Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital, 263129, India), Sonjoy Majumder(Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, 721302, India), Debanjan Bose(Department of Physics, Central University of Kashmir, , Ganderbal, 191131, Jammu & Kashmir, India) |
| Abstract: The detections of very-high-energy (VHE; $\gtrsim 100$~GeV) emission from GRB afterglows, highlighted by the exceptional brightness of GRB~221009A observed by LHAASO, indicate emission components beyond the standard electron synchrotron model. The presence of multi-TeV photons supports synchrotron self-Compton and potentially hadronic contributions, while the absence of coincident neutrinos in IceCube or KM3NeT places constraints on the microphysical parameters, jet kinetic energy, and ambient medium density. We model the VHE afterglow of GRB~221009A using an external forward shock from a Gaussian structured jet in a uniform density medium. This angular structure accounts for the extreme TeV output at an off-axis angle without requiring the energetics implied by a top-hat jet. We compute the associated $p\gamma$ neutrino flux in the PeV-EeV range and derive a time-integrated upper limit using the effective areas of upcoming neutrino detectors IceCube-Gen2 and GRAND200k, quantifying the contribution of individual GRBs to the expected neutrino events. For parameters inferred from the multi-wavelength spectral energy distribution, the predicted neutrino flux remains below the sensitivities of both detectors. Even for highly optimistic microphysical choices, our correlation analysis suggests that events from this GRB are of order $\sim 0.1$ for GRAND200k. We further compare on-axis and off-axis viewing geometries and find that jet geometry alone can change the predicted flux by nearly an order of magnitude. These results imply that a GRB that is both closer and brighter than GRB 221009A is required for realistic neutrino detection with upcoming facilities. Future CTA detections of GRBs will therefore be essential for constraining jet geometry, radiation mechanisms, and any associated neutrino emission.
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