Insights into the physics of gamma-ray bursts from the high-energy extension of their prompt emission spectra

Abstract

The study of the high-energy part (MeV-GeV) of the spectrum of gamma-ray bursts (GRBs) can play a crucial role in investigating the physics of prompt emission, but it is often hampered by low statistics and the paucity of GeV observations. In this work, we analyze the prompt emission spectra of the 22 brightest GRBs which have been simultaneously observed by Fermi/GBM and Fermi/LAT, spanning six orders of magnitude in energy. The high--energy photon spectra can be modeled with a power-law $N(E) beta $ possibly featuring an exponential cutoff. We find that, with the inclusion of the LAT data, the spectral index beta is softer than what is typically inferred from the analysis of Fermi/GBM data alone. Under the assumption that the emission is synchrotron, we derived a median value of the index $p 2.79$ of the power-law energy distribution of accelerated particles ($N( -p $). In nine out of 22 GRB spectra, we find a significant presence of an exponential cutoff at high energy, ranging between 14 and 298 MeV. By interpreting the observed cutoff as a sign of pair-production opacity, we estimate the jet bulk Lorentz factor Gamma , finding values in the range 130–330. These values are consistent with those inferred from the afterglow light curve onset time. Finally, by combining the information from the high-energy prompt emission spectrum with the afterglow light curve, we exploited a promising method to derive the distance $R$ from the central engine where the prompt emission occurs. The distances ($R > 10^ $ cm) inferred for the only two GRBs in our sample that are suitable for the application of this method, which have only lower limits on their cutoff energies, suggest large emitting regions, although they are still compatible with the standard model. Larger samples of GRBs with measured cutoff energies and afterglow deceleration time will allow for more informative values to be derived. These results highlight the importance of including high-energy data, when available, in the study of prompt spectra and their role in addressing the current challenges of the GRB standard model.

How do nuclear isomers influence the gamma-ray bursts in binary neutron star mergers?

“…The peak luminosity distribution of the long and short GRBs could be also fitted to a triple power law, implying that both types of GRBs could be produced by the same mechanism (Fraija et al, 2023b;Li et al, 2023). But if indeed the γ-ray emission took place in an optically thin region, far away from the central engine, the shock emission components are suppressed, and some other mechanisms may be at play (Ravasio et al, 2023). For example, recent simulations show that before the emergence on the jet from the neutron star remnant formed after the BNS merger, a UV/blue precursor signal is generated, that can "seed" the ultra relativistic GRB jet (Combi and Siegel, 2023).…”

“…Relativistic outflows are strongly beamed, such that the observer sees only the beaming angle, proportional to 1/Γ, where Γ is the Lorentz factor, a measure of the relativistic effects experienced by objects moving close to the speed of light. The estimated values for the Lorentz factor of the bulk matter in the jet are very large, between 100 and 1,000 (Ravasio et al, 2023). Particles accelerated to such relativistic speeds posses extremely high energy and emit synchrotron radiation in strong magnetic fields.…”