Lithium-6 and Gamma Rays: Complementary Constraints on Cosmic-Ray History

The rare isotope Li is made only by cosmic rays, dominantly in αα → Li fusion reactions with ISM helium. Consequently, this nuclide provides a unique diagnostic of the history of cosmic rays in our Galaxy. The same hadronic cosmic-ray interactions also produce high-energy γ rays (mostly via pp → π → γγ). Thus, hadronic γ-rays and Li are intimately linked. Specifically, Li directly encodes the local cosmic-ray fluence over cosmic time, while extragalactic hadronic γ rays encode an average cosmicray fluence over lines of sight out to the horizon. We examine this link and show how Li and γ-rays can be used together to place important model-independent limits on the cosmic-ray history of our Galaxy and the universe. We first constrain γ-ray production from ordinary Galactic cosmic rays, using the local Li abundance. We find that the solar Li abundance demands an accompanying extragalactic pionic γ-ray intensity which exceeds that of the entire observed EGRB by a factor of 2− 6. Possible explanations for this discrepancy are discussed. We then constrain Li production using recent determinations of extragalactic γ-ray background (EGRB). We note that cosmic rays created during cosmic structure formation would lead to pre-Galactic Li production, which would act as a “contaminant” to the primordial Li content of metal-poor halo stars; the EGRB can place an upper limit on this contamination if we attribute the entire EGRB pionic contribution to structure forming cosmic rays. Unfortunately, the uncertainties in the determination of the EGRB are so large that the present γ-ray data cannot guarantee that the pre-Galactic Li is small compared to primordial Li; thus, an improved determination of the EGRB will shed important new light on this issue. Our limits and their more model-dependent extensions will improve significantly with additional observations of Li in halo stars, and with improved measurements of the EGRB spectrum by GLAST. Subject headings: cosmic rays – gamma rays – nuclear reactions, nucleosynthesis, abundances also Department of Physics, University of Illinois