INTEGRAL detection of Galactic 60Fe gamma-ray line emission
25 June 2007By combining observations made during a period of two and a half years with the SPI instrument onboard INTEGRAL, a team headed by Roland Diehl has obtained the most significant detection to date of the 60Fe isotope in our Galaxy through its gamma-ray line emission.
Based on current stellar evolution models the iron isotope 60Fe is thought to be produced in supermassive stars in the final stages of their life. The high pressure and temperature conditions in these stars allow for the production of this radioactive heavy isotope. The supermassive stars end their lives in a supernova, enriching the interstellar medium with 60Fe along with other heavy elements and isotopes produced in the star's interior. Subsequent decay of the radioactive isotopes results in the emission of gamma-ray photons at specific energies. In the case of 60Fe, the decay half-life is ~1.5×106 years.
Another isotope, thought to be released into the interstellar medium mainly in periods earlier in a star's life and prior to the supernova, is 26Al. It decays with a half-life of 0.74×106 years. Unlike 26Al, for which numerous observations have confirmed the isotope's presence throughout our Galaxy from its diffuse gamma-ray emission, 60Fe was hitherto not unambiguously detected.
In an article by Wang et al. to be published in Astronomy & Astrophysics, the team presents the conclusive detection of gamma-ray line emission from 60Fe at the two characteristic energies 1173 keV and 1333 keV.
The two emission lines were observed in the dataset collected by the Spectrometer on INTEGRAL (SPI) of our Galaxy. Due to the weakness of the gamma-ray line emission from Galactic 60Fe, details of the line profile are not resolved, however, and the profile of the two emission lines is fitted with a Gaussian. By superimposing the two emission lines, the combined total emission from 60Fe is now determined with a higher accuracy than before - including the earlier publication by Harris et al.  that was based on one year of SPI observations.
Wang, W. et al., "SPI observations of the diffuse 60Fe emission in the Galaxy", to appear in A&A, arXiv:0704.3895v1 [astro-ph]
Harris, M.J., et al., "Detection of gamma-ray lines from interstellar 60Fe by the high resolution spectrometer SPI", A&A Vol. 433 (2005), pages L49-L52