Neutrino, Neutron, and Cosmic Ray Production in the External Shock Model of Gamma Ray Bursts

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Abstract
The hypothesis that ultra-high energy (> 10^{19} eV) cosmic rays (UHECRs) are accelerated by gamma-ray burst (GRB) blast waves is assumed to be correct. Implications of this assumption are then derived for the external shock model of gamma-ray bursts. The evolving synchrotron radiation spectrum in GRB blast waves provides target photons for the photomeson production of neutrinos and neutrons. Lower limits to the diffuse high-energy GRB neutrino background and the distribution of high-energy GRB neutrino events are derived. GRBs provide an intense flux of high-energy neutrons, with neutron-production efficiencies exceeding ~ 1% of the total energy release. The radiative characteristics of the neutron \beta-decay electrons are solved in a special case. Galaxies with GRB activity should be surrounded by radiation halos of ~ 100 kpc extent from the outflowing neutrons, consisting of a nonthermal optical/X-ray synchrotron component and a high-energy gamma-ray Compton component from upscattered microwave background radiation. The luminosity of sources of GRBs in the Milky Way must exceed ~ 2x10^{39} ergs/s to account for UHECR generation by GRBs. This figure is in accord with the burst rate implied by statistical modeling of GRBs in the external shock model, and represents ~5% of the CR power in the Galaxy. Relativistic fireball transients (FTs), which includes GRBs as a subclass, could account for a dominant contribution to CR production at energies << 10^{15} eV. This hypothesis for CR origin surmounts several observational difficulties in the conventional scenario that CRs originate from SNe that are unrelated to GRBs. FTs probably come from the >~ 60 M_\odot regime of stars that end their lives as Type Ib/c SNe, and leave \approx 10^6-10^7 medium mass (>~10-30 M_\odot) black holes in the galaxy. (abridged)

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