Study of $\mathrm{^{26}Al}$ in the COSI 2016 superpressure balloon flight

TL;DR

This paper reports on the 2016 balloon flight of COSI, a gamma-ray telescope, which detected a promising excess of $ m^{26}Al$ emission at 1.809 MeV with 3.7$\sigma$ significance, advancing understanding of stellar nucleosynthesis.

Contribution

First maximum-likelihood search for Galactic $ m^{26}Al$ in COSI 2016 data, revealing a significant excess and demonstrating COSI's capabilities in gamma-ray astronomy.

Findings

Detected a 3.7$\sigma$ excess of $ m^{26}Al$ emission

Measured flux of $(17.0 \pm 4.9) imes 10^{-4}$ ph cm$^{-2}$ s$^{-1}$

Validated COSI's potential for gamma-ray astrophysics

Abstract

The Compton Spectrometer and Imager (COSI) is a balloon-borne compact Compton telescope designed to survey the $\gamma$-ray sky from 0.2 to 5 MeV. COSI's wide field-of-view (FOV) and excellent energy resolution from high-purity germanium detectors make it uniquely capable of probing this under-explored energy regime. In particular, it can facilitate understanding of stellar nucleosynthesis through studies of diffuse emission from the radioisotope $\mathrm{^{26}Al}$ at 1.809 MeV. In 2016, COSI was launched from Wanaka, New Zealand on a NASA superpressure balloon and flew for 46 days. The flight was a technologic and scientific success, boasting live detection and polarization studies of GRB160530A, spectral analysis of the Crab Nebula and the 511-keV positron annihilation emission at the Galactic Center, and detection of Cygnus X-1. This article details the first maximum-likelihood search for the 1.809 MeV signature of Galactic $\mathrm{^{26}Al}$ in the 2016 data. The analysis reveals a promising excess around the expected energies of an $\mathrm{^{26}Al}$ signature with 3.7$\sigma$ significance and a measured flux of (17.0 $\pm$ 4.9) $\times$ 10$^{-4}$ ph cm$^{-2}$ s$^{-1}$. Further exploration is currently underway to solidify the measurement.