SPI observations of the diffuse 60Fe emission in the Galaxy

Aims. Gamma-ray line emission from radioactive decay of Fe-60 provides constraints on nucleosynthesis in massive stars and supernovae. Methods. spectrometer SPI on board INTEGRAL has accumulated nearly three years of data on gamma-ray emission from the Galactic plane. We have analyzed these data with suitable instrumental-background models and sky distributions to produce high-resolution spectra of Galactic emission. Results. We detect the gamma-ray lines from Fe-60 decay at 1173 and 1333 keV, obtaining an improvement over our earlier measurement of both lines with now 4.9s significance for the combination of the two lines. The average flux per line is ( 4.4 +/- 0.9) x 10(-5) ph cm(-2) s(-1) rad(-1) for the inner Galaxy region. Deriving the Galactic Al-26 gamma-ray line flux with using the same set of observations and analysis method, we determine the flux ratio of Fe-60/Al-26 gamma-rays as 0.148 +/- 0.06. Conclusions. The current theoretical predictions are consistent with our result. We discuss the implications of these results for the widely-held hypothesis that Fe-60 is synthesized in core-collapse supernovae, and also for the closely-related question of the precise origin of Al-26 in massive stars.