X-ray Flares in Gamma-Ray Bursts at High and Very-High-Energies

Pawan Tiwari; Biswajit Banerjee; Gor Oganesyan; Ansh Chopra; Annarita Ierardi; Marica Branchesi; Stefano Ascenzi; Maria Edvige Ravasio

arXiv · 2609.00684

X-ray Flares in Gamma-Ray Bursts at High and Very-High-Energies

Pawan Tiwari·Biswajit Banerjee·Gor Oganesyan·Ansh Chopra·Annarita Ierardi·Marica Branchesi·Stefano Ascenzi·Maria Edvige Ravasio

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Abstract

A significant fraction of GRB early afterglows exhibit fast and bright X-ray flares, discovered by Swift X-ray Telescope. Their rapid temporal variability and spectral evolution have suggested an internal shock origin, analogous to prompt MeV emission. However, their physical origin and radiation mechanism remain debated. High-energy (> 100 MeV) and very-high-energy (> 30 GeV) gamma-ray observations provide a powerful probe of flare dissipation region, constraining its size, magnetic field strength, and particle acceleration under the synchrotron self-Compton scenario in optically thin relativistic jets. We present a systematic multi-wavelength study of 66 X-ray flares from 47 GRBs observed by X-ray Telescope over 17 years, all within field of view of Fermi Large Area Telescope. We investigate their GeV counterparts and find that only five flares exhibit significant high-energy emission (> 3 sigma). Broadband spectral modeling indicates that this GeV emission is consistent with standard forward shock afterglow. We further investigate correlations between flare spectral properties and energy fluxes at 1 keV, 10 keV, and 1 GeV. Using a synchrotron self-Compton model, we constrain the physical conditions of the emitting region, including magnetic field strength, bulk Lorentz factor, and emission radius. For the most stringent GeV upper limits, we find a magnetic-to-electron luminosity ratio greater than or equal to 1, implying a highly magnetized emitting region. We predict very-high-energy gamma-ray emission from X-ray flares at early (~500s) and late (~5000s) times and assess their detectability with Cherenkov Telescopes. We find that later X-ray flares provide the most promising targets for follow-up observations owing to improved observational accessibility, sensitivity, and reduced response-time constraints of facilities such as Cherenkov Telescope Array Observatory.

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Pawan Tiwari, Biswajit Banerjee, Gor Oganesyan, Ansh Chopra, Annarita Ierardi, Marica Branchesi, Stefano Ascenzi, Maria Edvige Ravasio. 2026-09-01. X-ray Flares in Gamma-Ray Bursts at High and Very-High-Energies. https://arxiv.org/abs/2609.00684

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