Acceleration of Ultra-High Energy Cosmic Rays in the Colliding Shells of Blazars and GRBs: Constraints from the Fermi Gamma ray Space Telescope

TL;DR

This paper uses Fermi Telescope data to constrain how ultra-high energy cosmic rays are accelerated in blazars and GRBs, suggesting Fe nuclei are more likely than protons in AGNs and identifying likely sources of these cosmic rays.

Contribution

It provides new constraints on maximum cosmic-ray energies in colliding shells of GRBs and blazars based on Fermi observations, highlighting Fe nuclei acceleration in AGNs.

Findings

Fe nuclei are more likely to reach ultra-high energies in AGNs.

Powerful GRBs can accelerate protons and Fe nuclei above 10^{20} eV.

BL Lac objects and FR1 radio galaxies are favored as UHECR sources.

Abstract

Fermi Gamma ray Space Telescope measurements of spectra, variability time scale, and maximum photon energy give lower limits to the apparent jet powers and, through gammagamma opacity arguments, the bulk Lorentz factors of relativistic jets. The maximum cosmic-ray particle energy is limited by these two quantities in Fermi acceleration scenarios. Recent data are used to constrain the maximum energies of cosmic-ray protons and Fe nuclei accelerated in colliding shells of GRBs and blazars. The Fermi results indicate that Fe rather than protons are more likely to be accelerated to ultra-high energies in AGNs, whereas powerful GRBs can accelerate both protons and Fe to >~ 10^{20} eV. Emissivity of nonthermal radiation from radio galaxies and blazars is estimated from the First Fermi AGN Catalog, and shown to favor BL Lac objects and FR1 radio galaxies over flat spectrum radio quasars, FR2 radio galaxies, and long-duration GRBs as the sources of UHECRs.