Study of 26 Al in the COSI 2016 superpressure balloon flight

The Compton Spectrometer and Imager (COSI) is a balloon-borne compact Compton telescope designed to survey the W-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 26Al 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 26Al in the 2016 data. The analysis reveals a promising excess around the expected energies of an 26Al signature with 3.7f significance and a measured flux of (17.0 ± 4.9) × 10−4 ph cm−2 s−1. Further exploration is currently underway to solidify the measurement.