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Soft gamma-ray polarimetry with COSI

Presentation #204.05 in the session “Missions and Instruments (Oral)”.

Published onApr 01, 2022
Soft gamma-ray polarimetry with COSI

The Compton Spectrometer and Imager (COSI) is Compton telescope and polarimeter recently selected by NASA as a Small Explorer (SMEX) mission. Its polarimetry capabilities were verified during a balloon campaign in 2016. Measurements of the linear polarization of high-energy emission from pulsars, accreting black holes, and gamma-ray bursts (GRBs) provide an opportunity for constraining the emission mechanisms and geometries in the sources. For photons in the soft (MeV) gamma-ray band, Compton scattering is the most likely interaction to occur in detectors. Compton telescopes detect multiple interactions from individual incoming photons, allowing for polarization information to be captured through measurements of the distribution of azimuthal angles. While the standard method relies on binning the photons to produce and fit an azimuthal scattering angle distribution, improved polarization sensitivity is obtained by using additional information to more accurately weight each event’s contribution to the likelihood statistic. This talk describes COSI’s capabilities for polarization measurements. It also details the maximum likelihood method (MLM) used for past polarization measurements and a joint spectral-polarization MLM tool for future observations.

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