Gamma-Ray Limits on Galactic [TSUP]60[/TSUP]F[CLC]e[/CLC] Nucleosynthesis and Implications on the Origin of the [TSUP]26[/TSUP]A[CLC]l[/CLC] Emission

Abstract

The Gamma-Ray Imaging Spectrometer recently observed the gamma-ray emission from the Galactic center region. We have detected the 1809 keV Galactic 26Al emission at a significance level of 6.8 σ but have found no evidence for emission at 1173 and 1332 keV, which is expected from the decay chain of the nucleosynthetic 60Fe. The isotopic abundances and fluxes are derived for different source distribution models. The resulting abundances are between 2.6±0.4 and 4.5±0.7 M for 26Al, and a 2 σ upper limit for 60Fe is between 1.7 and 3.1 M. The measured 26Al emission flux is significantly higher than that derived from the Compton Gamma Ray Observatory COMPTEL 1.8 MeV sky map. This suggests that a fraction of the 26Al emission may come from extended sources with low surface brightness that are invisible to COMPTEL. We obtain an60Fe/26Al flux ratio 2 σ upper limit of 0.14, which is slightly lower than the 0.16 predicted from current nucleosynthesis models assuming that Type II supernovae are the major contributors to the Galactic 26Al. Since the uncertainties in the predicted fluxes are large (up to a factor of 2), our measurement is still compatible with the theoretical expectations.