The Compton Spectrometer and Imager (COSI)

The Compton Spectrometer and Imager (COSI) is a balloon-borne, soft gamma-ray (0.2-5 MeV) telescope designed to study astrophysical sources of nuclear-line emission and gamma-ray polarization. The heart of COSI is a compact array of cross-strip germanium detectors (GeDs), providing excellent spectral resolution (0.3% at 662 keV) and the capability of tracking the photon scattering history with full 3D position resolution of less than 2 $mm^{3}$ for each interaction. With good efficiency, wide-field imaging, and polarization sensitivity, COSI is a powerful observatory in the medium gamma-ray regime. The most recent balloon flight of COSI was launched from Wanaka, New Zealand, in May 2016 on a Super Pressure Balloon floating for 46 days. During this flight, COSI discovered GRB 160530A and detected several sources, including the Crab, Cen A, Cyg X-1, and the 511-keV emission from the galactic center. Here we will present the characteristics and capabilities of this novel instrument and preliminary results from the 2016 flight.