The upcoming balloon campaign of the Compton Spectrometer and Imager (COSI)

Abstract The Compton Spectrometer and Imager (COSI), formerly known as the Nuclear Compton Telescope (NCT), is a balloon-borne soft gamma-ray telescope (0.2–5 MeV) 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 ( ~ 0.2 – 1 % ) and the capability to track individual photon interactions with full 3D position resolution to 1.6 mm 3 . COSI is built upon considerable heritage from the previous NCT balloon instrument, which has flown successfully on two conventional balloon flights to date. The Crab Nebula was detected at a significance of 6 σ in the second flight, which is the first reported detection of an astrophysical source by a compact Compton telescope. COSI has been upgraded from the previous NCT instrument to be an Ultra Long Duration Balloon (ULDB) payload, utilizing a new detector configuration optimized for polarization sensitivity and employing a mechanical cryocooler to remove consumables (LN2) for ULDB flights. The instrument is being integrated for a ULDB flight in December 2014 from Antarctica on a superpressure balloon. Here we will present the redesign of the instrument and our current progress in preparing for the flight.