A marginally fast-cooling proton-synchrotron model for prompt GRBs

A small fraction of gamma-ray bursts (GRBs) with available data down to soft X-rays (similar to 0.5 keV) has been shown to feature a spectral break in the low-energy part (similar to 1-10 keV) of their prompt emission spectrum. The overall spectral shape is consistent with optically thin synchrotron emission from a population of particles that have cooled on a time-scale comparable to the dynamic time to energies that are still much higher than their rest-mass energy (marginally fast cooling regime). We consider a hadronic scenario and investigate if the prompt emission of these GRBs can originate from relativistic protons that radiate synchrotron in the marginally fast cooling regime. Using semi-analytical methods, we derive the source parameters, such as magnetic field strength and proton luminosity, and calculate the high-energy neutrino emission expected in this scenario. We also investigate how the emission of secondary pairs produced by photopion interactions and gamma gamma pair production affect the broad-band photon spectrum. We support our findings with detailed numerical calculations. Strong modification of the photon spectrum below the break energy due to the synchrotron emission of secondary pairs is found, unless the bulk Lorentz factor is very large (Gamma greater than or similar to 10(3)). Moreover, this scenario predicts unreasonably high Poynting luminosities because of the strong magnetic fields (10(6)-10(7) G) that are necessary for the incomplete proton cooling. Our results strongly disfavour marginally fast cooling protons as an explanation of the low-energy spectral break in the prompt GRB spectra.

Automated summary

This study examines the prompt emission of a small fraction of gamma-ray bursts (GRBs) and investigates the possibility of relativistic protons as the source. The results strongly disfavor marginally fast cooling protons as an explanation for the low-energy spectral break in prompt GRB spectra.