Limitations on the Photo-disintegration Process as a Source of VHE Photons

We consider whether photo-disintegration is ever able to provide an effective mechanism for the production of VHE $\gamma$-ray emission from astrophysical sources. We find that the efficiency of this process is always smaller by a factor $A/Z^{2}$ ($\sim 4/A$) than that of nuclei cooling through Bethe-Heitler pair-production. Furthermore, for sources optically thin to TeV emission, we find that the efficiency of this process can be no more than $3\times 10^{-5}(R_{\rm source}/R_{\rm Larmor})$, where $R_{\rm source}$ is the source size and $R_{\rm Larmor}$ is the CR nuclei Larmor radius. We conclude that this process is unable to provide an effective mechanism for VHE $\gamma$-ray emission from astrophysical sources.Comment: 10 pages, 5 figure

Cosmic Rays in a Galactic Breeze

Motivated by the discovery of the non-thermal Fermi bubble features both below and above the Galactic plane, we investigate a scenario in which these bubbles are formed through Galacto-centric outflow. Cosmic rays (CR) both diffusing and advecting within a Galactic breeze outflow, interacting with the ambient gas present, give rise to gamma-ray emission, providing an approximately flat surface brightness profile of this emission, as observed. Applying the same outflow profile further out within the disk, the resultant effects on the observable CR spectral properties are determined. A hardening in the spectra due to the competition of advective and diffusive propagation within a particular energy range is noted, even in the limiting case of equal CR diffusion coefficients in the disk and halo. It is postulated that this hardening effect may relate to the observed hardening feature in the CR spectrum at a rigidity of $\approx 200$ GV.Comment: 8 pages (2 columns), 5 figures. Published in Physical Review

A Galactic Halo Origin of the Neutrinos Detected by IceCube

Recent IceCube results suggest that the first detection of very high energy astrophysical neutrinos have been accomplished. We consider these results at face value in a Galactic origin context. Emission scenarios from both the Fermi bubble and broader halo region are considered. We motivate that such an intensity of diffuse neutrino emission could be Galactic in origin if it is produced from an outflow into the halo region. This scenario requires cosmic ray transport within the outflow environment to be different to that inferred locally within the disk and that activity in the central part of the Galaxy accelerates cosmic rays to trans-"knee" energies before they escape into an outflow. The presence of a large reservoir of gas in a very extended halo around the Galaxy, recently inferred from X-ray observations, implies that relatively modest acceleration power of $10^{39}$ erg s$^{-1}$ in PeV energy cosmic rays may be sufficient to explain the observed neutrino flux. Such a luminosity is compatible with that required to explain the observed intensity of cosmic rays around the "knee".Comment: 7 pages, 2 figure

Cosmogenic photons as a test of ultra-high energy cosmic ray composition

Although recent measurements of the shower profiles of ultra-high energy cosmic rays suggest that they are largely initiated by heavy nuclei, such conclusions rely on hadronic interaction models which have large uncertainties. We investigate an alternative test of cosmic ray composition which is based on the observation of ultra-high energy photons produced through cosmic ray interactions with diffuse low energy photon backgrounds during intergalactic propagation. We show that if the ultra-high energy cosmic rays are dominated by heavy nuclei, the flux of these photons is suppressed by approximately an order of magnitude relative to the proton-dominated case. Future observations by the Pierre Auger Observatory may be able to use this observable to constrain the composition of the primaries, thus providing an important cross-check of hadronic interaction models.Comment: 4 pages, 2 figure