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

Abstract

The rare isotope 6Li is made only by cosmic rays, predominantly in αα → 6Li fusion reactions with interstellar medium (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 → π0 → γγ). Thus, hadronic γ-rays and 6Li are intimately linked. Specifically, 6Li directly encodes the local cosmic-ray fluence over cosmic time, while extragalactic hadronic γ-rays encode an average cosmic-ray fluence over lines of sight out to the horizon. We examine this link and show how 6Li 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 6Li abundance. We find that the solar 6Li abundance demands an accompanying extragalactic pionic γ-ray intensity that exceeds that of the entire observed extragalactic γ-ray background (EGRB) by a factor of 2-6. Possible explanations for this discrepancy are discussed. We then constrain Li production using recent determinations of the 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 7Li 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 contribution is small compared to primordial 7Li; 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 6Li in halo stars and with improved measurements of the EGRB spectrum by GLAST.