Measurement of Galactic 26Al with the Compton Spectrometer and Imager

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