UHECR from High- and Low-Luminosity Gamma-Ray Bursts

For a long time Gamma-Ray Bursts (GRBs) have been discussed as potential origin of Ultra-High-Energy Cosmic Rays (UHECRs). This paradigm is challenged by the lack of High-Energy (HE) neutrinos detected by IceCube that could be associated with known GRBs. In this work, we study two scenarios that could reconcile GRBs as UHECR sources: First we fit the UHECR spectrum and < X-max > as observed by Auger with conventional long-duration GRBs in a multi-zone internal shock model. Here, we systematically scan over different engine realisations that may be distinguished by their light curves. We show that a good fit is possible for a broad range of parameters if sigma(X-max) is not included and that the corresponding neutrino fluxes are in reach of future instruments such as IceCube Gen2. In this context, we critically review the required engine kinetic energy. Second, we explore the prompt phase of Low-Luminosity GRBs (LL-GRBs) as potential sources of Very-High-Energy (VHE) gamma-rays and UHECR. For this purpose, we model a LL-GRB with properties similar to GRB 980425 within the internal shock model and vary the fraction of internal energy supplying the magnetic field. We find that for strong magnetic fields, iron nuclei (protons) can be accelerated up to 1011 GeV (10(9) GeV), while for low magnetic fields a VHE component potentially observable by Imaging Air Cherenkov Telescopes (IACTs) could be produced.