Measurement of Galactic ${}^{26}$Al 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 $\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 ($|\ell |\le 30^{\circ }$, $|b|\le 10^{\circ }$) 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.

23 pages, 22 figures, accepted by The Astrophysical Journal (ApJ)

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Cited by in corpus (7)

• Calibrations of the Compton Spectrometer and Imager
• Probing the Galactic Diffuse Continuum Emission with COSI
• ${}^{26}$Al gamma rays from the Galaxy with INTEGRAL/SPI
• Evaluation of the performance of a CdZnTe-based soft $\gamma$-ray detector for CubeSat payloads
• Enhancing Compton telescope imaging with maximum a posteriori estimation: a modified Richardson-Lucy algorithm for the Compton Spectrometer and Imager
• Imaging and Spectral Fitting of Bright Gamma-ray Sources with the COSI Balloon Payload
• Performance Evaluation of a silicon-based 6U Cubesat detector for soft $\gamma$-ray astronomy