Measurement of Galactic 26Al with the Compton Spectrometer and Imager

Abstract

The Compton Spectrometer and Imager (COSI) is a balloon-borne compact Compton telescope designed to survey the 0.2–5 MeV sky. COSI’s energy resolution of ∼0.2% at 1.8 MeV, single-photon reconstruction, and wide field of view make it capable of studying astrophysical nuclear lines, particularly the 1809 keV γ -ray line from decaying Galactic 26 Al. Most 26 Al originates in massive stars and core-collapse supernova nucleosynthesis, but the path from stellar evolution models to Galaxy-wide emission remains unconstrained. In 2016, COSI had a successful 46 day flight on a NASA superpressure balloon. Here, we detail the first search for the 1809 keV 26 Al line in the COSI 2016 balloon flight using a maximum-likelihood analysis. We find a Galactic 26 Al flux of (8.6 ± 2.5) × 10 −4 ph cm −2 s −1 within the Inner Galaxy (∣ ℓ ∣ ≤ 30°, ∣ b ∣ ≤ 10°) with 3.7 σ significance above background. Within uncertainties, this flux is consistent with expectations from previous measurements by SPectrometer on INTEGRAL (SPI) and the Compton Telescope on the Compton Gamma-Ray Observatory (COMPTEL). This analysis demonstrates COSI’s powerful capabilities for studies of γ -ray lines and underscores the scientific potential of future compact Compton telescopes. In particular, the next iteration of COSI as a NASA Small Explorer satellite has recently been approved for launch in 2025.