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arXiv · 2202.11713

Measurement of Galactic $^{26}$Al with the Compton Spectrometer and Imager

Jacqueline Beechert·Thomas Siegert·John A. Tomsick·Andreas Zoglauer·Steven E. Boggs·Terri J. Brandt·Hannah Gulick·Pierre Jean·Carolyn Kierans·Hadar Lazar·Alexander Lowell·Jarred M. Roberts·Clio Sleator·Peter von Ballmoos
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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 $\sim$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 $\gamma$-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 $(8.6 \pm 2.5) \times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ within the Inner Galaxy ($|\ell| \leq 30^{\circ}$, $|b| \leq 10^{\circ}$) with 3.7$\sigma$ 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 $\gamma$-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.

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Jacqueline Beechert, Thomas Siegert, John A. Tomsick, Andreas Zoglauer, Steven E. Boggs, Terri J. Brandt, Hannah Gulick, Pierre Jean, Carolyn Kierans, Hadar Lazar, Alexander Lowell, Jarred M. Roberts, Clio Sleator, Peter von Ballmoos. 2022-02-23. Measurement of Galactic $^{26}$Al with the Compton Spectrometer and Imager. https://doi.org/10.3847/1538-4357/ac56dc

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