6Li and Gamma Rays: Complementary Constraints on Cosmic‐Ray History

The rare isotope 6Li is made only by cosmic rays, dominantly in alpha+alphafusion reactions with ISM helium. Consequently, this nuclide provides a uniquediagnostic of the history of cosmic rays in our Galaxy. The same hadroniccosmic-ray interactions also produce high-energy gamma rays (mostly via neutralpion production). Thus, hadronic gamma-rays and 6Li are intimately linked.Specifically, 6Li directly encodes the local cosmic-ray fluence over cosmictime, while extragalactic hadronic gamma rays encode an average cosmic-rayfluence over lines of sight out to the horizon. We examine this link and showhow 6Li and gamma-rays can be used together to place importantmodel-independent limits on the cosmic-ray history of our Galaxy and theuniverse. We first constrain gamma-ray production from ordinary Galactic cosmicrays, using the local 6Li abundance. We find that the solar 6Li abundancedemands an accompanying extragalactic pionic gamma-ray intensity which exceedsthat of the entire observed EGRB by a factor of 2-6. Possible explanations forthis discrepancy are discussed. We then constrain Li production using recentdeterminations of extragalactic gamma-ray background (EGRB). We note thatcosmic rays created during cosmic structure formation would lead topre-Galactic Li production, which would act as a "contaminant" to theprimordial 7Li content of metal-poor halo stars. We find the uncertainties inthe observed EGRB are so large that we cannot exclude a pre-Galactic Li whichis comparable to primordial 7Li. Our limits and their more model-dependentextensions will improve significantly with additional observations of 6Li inhalo stars, and with improved measurements of the EGRB spectrum by GLAST.(Abriged abstract)Comment: 24 pages, 1 figure, AASTeX. Comments welcom