Data Analysis for a Balloon Borne Compton Polarimeter

Abstract

The Gamma RAy Polarimeter Experiment (GRAPE), a balloon borne Compton polarimeter for 50-500 keV gamma rays, was successfully flown for the second time in 2014. GRAPE consists of 24 collimated polarimeter modules. Each module is made up of 64 rectangular scintillators elements in a grid of 8 $\times$ 8 (36 plastic scintillators surrounded by 28 CsI scintillators). GRAPE uses photon scattered events between 2 detector elements to measure the polarization of the measured photons. GRAPE was flown from Fort Sumner, New Mexico on the morning of September 26$^{\text{th}}$ 2014. GRAPE was at float altitude for 14.4 hours and the Crab (primary target) was observed for 1.8 hours (initially planned for 8 hours). Background dominates the observations at flight altitudes, which depends on many flight and instrument parameters (altitude, instrument pointing, temperatures, etc). A technique based on the Principle Component Analysis (PCA) was implemented which used the varying parameters to estimate the background for the Crab. Our instrument design allows for an optical crosstalk (a known issue) that arises from light exiting the scintillator element and spilling to neighboring anodes. A model was developed to represent the crosstalk, which was incorporated in our simulations and the instrument response. The analysis was focused on the phase-integrated data (due to limited statistics of the shortened observation period). A power-law spectrum with photon index of 1.70$\pm$0.24 and a normalization of 1.01$\pm$1.35 ph/keV/s/cm$^2$ was measured. A polarization fraction of 0.43$\pm$0.4 and a polarization angle of 56$^\circ \pm$30$^\circ$ was measured for the phase integrated Crab observation in the 70-200 keV energy range. This result was not sufficiently significant enough to further our understanding of the emission mechanism.