SPI observations of the diffuse ⁶⁰Fe emission in the Galaxy

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany e-mail: wwang@mpe.mpg.de
² AstroParticule et Cosmologie (APC), 11 place Marcelin Berthelot, 75231 Paris, France
³ DSM/DAPNIA/SAp, CEA Saclay, 91191 Gif-sur-Yvette, France
⁴ Centre d'tude Spatiale des Rayonnements, BP 4346, 31028 Toulouse Cedex 4, France
⁵ Space Sciences Laboratory, UC Berkeley, CA 94720, USA

Received: 20 December 2006
Accepted: 26 April 2007

Abstract

Aims.Gamma-ray line emission from radioactive decay of ⁶⁰Fe 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 γ-ray lines from ⁶⁰Fe decay at 1173 and 1333 keV, obtaining an improvement over our earlier measurement of both lines with now 4.9σ significance for the combination of the two lines. The average flux per line is (4.4 ± 0.9)  10^-5 ph cm^-2 s^-1 rad^-1 for the inner Galaxy region. Deriving the Galactic ²⁶Al gamma-ray line flux with using the same set of observations and analysis method, we determine the flux ratio of ⁶⁰Fe/²⁶Al 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 ⁶⁰Fe is synthesized in core-collapse supernovae, and also for the closely-related question of the precise origin of ²⁶Al in massive stars.