Detection of γ-ray lines from interstellar $\mathsf{^{60}}$Fe by the high resolution spectrometer SPI

Abstract

It is believed that core-collapse supernovae (CCSN), occurring at a rate ∼once per century, have seeded the inter- stellar medium with long-lived radioisotopes such as 60 Fe (half-life 1.5 Myr), which can be detected by the γ-rays emitted when they β-decay. Here we report the detection of the 60 Fe decay lines at 1173 keV and 1333 keV with fluxes 3.7 ± 1.1 × 10 −5 γ 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 26 Al yielded a line flux ratio 60 Fe/ 26 Al = 0.11 ± 0.03. This supports the hypothesis that there is an extra source of 26 Al in addition to CCSN.