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 26Al. Most 26Al 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 26Al line in the COSI 2016 balloon flight using a maximum likelihood analysis. We find a Galactic 26Al flux of (8.6±2.5)×10−4 ph cm−2 s−1 within the Inner Galaxy (∣ℓ∣≤30∘, ∣b∣≤10∘) with 3.7σ significance above background. Within uncertainties, this flux is consistent with expectations from previous measurements by SPI and 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.
@article{arxiv.2202.11713,
title = {Measurement of Galactic $^{26}$Al with the Compton Spectrometer and Imager},
author = {Jacqueline Beechert and Thomas Siegert and John A. Tomsick and Andreas Zoglauer and Steven E. Boggs and Terri J. Brandt and Hannah Gulick and Pierre Jean and Carolyn Kierans and Hadar Lazar and Alexander Lowell and Jarred M. Roberts and Clio Sleator and Peter von Ballmoos},
journal= {arXiv preprint arXiv:2202.11713},
year = {2022}
}
Comments
23 pages, 22 figures, accepted by The Astrophysical Journal (ApJ)