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

Abstract

The rare isotope 6 Li is made only by cosmic rays, dominantly in αα → 6 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 0 → γγ). Thus, hadronic γ-rays and 6 Li are intimately linked. Specifically, 6 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 6 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 6 Li abundance. We find that the solar 6 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 7 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 7 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 6 Li in halo stars, and with improved measurements of the EGRB spectrum by GLAST.

Can Galactic Cosmic Rays Account for Solar 6 Li without Overproducing Gamma Rays?

“…In Fields & Prodanović (2005), we formally demonstrated and quantified the tight connection between CR lithium synthesis and hadronic g-ray production. We showed that both observables are a measure of the time-integrated CR flux (fluence F).…”

The Search for Extragalactic Lithium Hydride

“…The lithium problem and possible solutions have recently been reviewed by Fields (2011) and Pospelov & Pradler (2010). A serious possibility is that nonstandard processes occurred during primordial nucleosynthesis, altering the lithium abundance; dark matter decays can do this (Jedamzik et al 2006).…”

Parsec-scale Variations in the 7Li i/6Li i Isotope Ratio Toward IC 348 and the Perseus OB 2 Association* †

“…The observed 7 Li/ 6 Li ∼ 2 as theoretically predicted (MAR) occurs through both alpha-alpha fusion reactions with interstellar He (Fields & Prodanovic 2005) and cosmic rays bombarding C, N, and O nuclei in the interstellar medium surrounding the IC 348 region. Cosmic ray bombardment of the interstellar medium also creates Be and B.…”