Gamma-ray limits on Galactic 60Fe nucleosynthesis and implications on the Origin of the 26Al emission

Juan E. Naya(1,3), Scott D. Barthelmy(1,3), Lyle M. Bartlett(1,4), Neil Gehrels(1), Ann Parsons(1), Bonnard J. Teegarden(1), Jack Tueller(1), Marvin Leventhal(2)


(1) NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
(2) Dept. of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
(3) Universities Space Research Association, 7501 Forbes Blvd. #206, Seabrook, MD 20706-2253, USA
(4) NAS/NRC Resident Research Associate, Code 718, NASA/GSFC, Greenbelt, MD 20771, USA

Paper: ApJ Letters, in press

Abstract:

The Gamma Ray Imaging Spectrometer (GRIS) 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 sigma but have found no evidence for emission at 1173 keV and 1332 keV, 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 Mo for 26Al and a 2 sigma upper limit for 60Fe between 1.7 and 3.1 Mo. The measured 26Al emission flux is significantly higher than that derived from the CGRO/COMPTEL 1.8 MeV sky map. This suggests that a fraction of the 26Al emission may come from extended sources with a low surface brightness that are invisible to COMPTEL. We obtain a 60Fe to 26Al flux ratio 2 sigma upper limit of 0.14, which is slightly lower than the 0.16 predicted from current nucleosynthesis models assuming that SNII 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.

Preprints available from the authors at naya@tgrs2.gsfc.nasa.gov , or the raw TeX (no figures) if you click here.

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