Detection of γ-ray lines from interstellar 60Fe by the high resolution spectrometer SPI

It is believed that core- collapse supernovae ( CCSN), occurring at a rate similar to once per century, have seeded the interstellar medium with long- lived radioisotopes such as Fe-60 ( half- life 1.5 Myr), which can be detected by the gamma- rays emitted when they beta- decay. Here we report the detection of the Fe-60 decay lines at 1173 keV and 1333 keV with fluxes 3.7 +/- 1.1 x 10(-5) gamma cm(-2) s(-1) per line, in spectra taken by the SPI spectrometer on board INTEGRAL during its first year. The same analysis applied to the 1809 keV line of Al-26 yielded a line flux ratio Fe-60/Al-26 = 0.11 +/- 0.03. This supports the hypothesis that there is an extra source of Al-26 in addition to CCSN.