The Limited Contribution of Low- and High-Luminosity Gamma-Ray Bursts to Ultra-High Energy Cosmic Rays

Filip Samuelsson, Damien Bégué, Felix Ryde, Asaf Pe'er

Published 2018-10-15Version 1

The acceleration site for ultra-high energy cosmic rays is still an open question despite extended research. In this paper, we reconsider the prompt phase of gamma-ray bursts as a possible candidate for this acceleration and constrain the maximum proton energy in optically thin synchrotron and photospheric models, using properties of the prompt photon spectra. We find that neither of the models favour acceleration of protons to $10^{20}$ eV in high-luminosity bursts. We repeat the calculations for low-luminosity gamma-ray bursts considering both protons and completely stripped iron and find that the highest obtainable energies are $< 10^{19}$ eV and $< 10^{20}$ eV for protons and iron respectively, regardless of the model. We conclude therefore that, gamma-ray bursts, including low-luminosity bursts, contribute little to none to the ultra-high energy cosmic rays. Any association between ultra-high energy cosmic rays and gamma-ray bursts would require very efficient acceleration, favour photospheric origin of the prompt gamma-rays, and result in high prompt optical spectral fluxes.