Ultra-High Energy Cosmic Rays and Neutron-Decay Halos from Gamma Ray Bursts

Simple arguments concerning power and acceleration efficiency show that ultra-high energy cosmic rays (UHECRS) with energies >~ 10^{19} eV could originate from GRBs. Neutrons formed through photo-pion production processes in GRB blast waves leave the acceleration site and travel through intergalactic space, where they decay and inject a very energetic proton and electron component into intergalactic space. The neutron-decay protons form a component of the UHECRs, whereas the neutron-decay electrons produce optical/X-ray synchrotron and gamma radiation from Compton-scattered background radiation. A significant fraction of galaxies with GRB activity should be surrounded by neutron-decay halos of characteristic size ~ 100 kpc.Comment: 3 pages, in 2nd Rome Workshop on Gamma Ray Bursts in the Afterglow Era (17-20 October 2000

On Hadronic Models for the Anomalous γ\gamma-ray Emission Component in GRB 941017

Gonz\'alez et al. (2003) have reported the discovery of an anomalous radiation component from ~ 1 -- 200 MeV in GRB 941017. This component varies independently of and contains >~ 3 times the energy found in the prompt ~ 50 keV -- 1 MeV radiation component that is well described by the relativistic synchrotron-shock model. Acceleration of hadrons to very high energies by GRBs could give rise to a separate emission component. Two models, both involving acceleration of ultra-high energy cosmic rays with subsequent photomeson interactions, are considered. The first involves a pair-photon cascade initiated by photohadronic processes in the GRB blast wave. Calculations indicate that the cascade produces a spectrum that is too soft to explain the observations. A second model is proposed where photopion interactions in the GRB blast-wave shell give rise to an escaping collimated neutron beam. The outflowing neutrons undergo further photopion interactions to produce a beam of hyper-relativistic electrons that can lose most of their energy during a fraction of a gyroperiod in the Gauss-strength magnetic fields found in the circumburst medium. This secondary electron beam produces a hard synchrotron radiation spectrum that could explain the anomalous component in GRB 941017.Comment: 5 pages, 1 figure, in 2003 Santa Fe Conference on GRB

Gamma Rays from Cosmic Rays in Supernova Remnants

Context: Cosmic rays are thought to be accelerated at supernova remnant (SNR) shocks, but conclusive evidence is lacking. Aims: New data from ground-based gamma-ray telescopes and the Large Area Telescope on the Fermi Gamma-ray Space Telescope are used to test this hypothesis. A simple model for gamma-ray production efficiency is compared with measured gamma-ray luminosities of SNRs, and the GeV to TeV fluxes ratios of SNRs are examined for correlations with SNR ages. Methods: The supernova explosion is modeled as an expanding spherical shell of material that sweeps up matter from the surrounding interstellar medium (ISM). The accumulated kinetic energy of the shell, which provides the energy available for nonthermal particle acceleration, changes when matter is swept up from the ISM and the SNR shell decelerates. A fraction of this energy is assumed to be converted into the energy of cosmic-ray electrons or protons. Three different particle radiation processes---nuclear pion-production interactions, nonthermal electron bremsstrahlung, and Compton scattering---are considered. Results: The efficiencies for gamma-ray production by these three processes are compared with gamma-ray luminosities of SNRs. Our results suggest that SNRs become less gamma-ray luminous at >~ 10^4 yr, and are consistent with the hypothesis that supernova remnants accelerate cosmic rays with an efficiency of ~10% for the dissipation of kinetic energy into nonthermal cosmic rays. Weak evidence for an increasing GeV to TeV flux ratio with SNR age is found.Comment: 5 pgs, 3 figs, Astronomy and Astrophysics research note, in pres

Neutrinos and Gamma Rays from Photomeson Processes in Gamma Ray Bursts

Acceleration of high-energy hadrons in GRB blast waves will be established if high-energy neutrinos are detected from GRBs. Recent calculations of photomeson neutrino production are reviewed, and new calculations of high-energy neutrinos and the accompanying hadronic cascade radiation are presented. If hadrons are injected in GRB blast waves with an energy corresponding to the measured hard X-ray/soft gamma-ray emission, then only the most powerful bursts at fluence levels >~ 3e-4 erg cm^{-2} offer a realistic prospect for detection of muon neutrinos. Detection of high-energy neutrinos are likely if GRB blast waves have large baryon loads and Doppler factors <~ 200. Significant limitations on the hadronic baryon loading and the number of expected neutrinos are imposed by the fluxes from pair-photon cascades initiated in the same processes that produce neutrinos.Comment: 4 pages, 2 figures, in 2003 Santa Fe Conference on GRB