Date of Award
Program or Major
Doctor of Philosophy
Mark L McConnell
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 uses events in which a photon scatters between 2 detector elements to measure the polarization of the measured photons. GRAPE consists of 24 collimated polarimeter modules. Each module is made up of 64 rectangular scintillators elements in a grid of 8 × 8 (36 plastic scintillators surrounded by 28 CsI scintillators). GRAPE was flown from Fort Sumner, New Mexico on the morning of September 26th 2014. The experiment was at float altitude for 14.4 hours. The Crab (the pulsar and the nebula) was the primary target of the experiment. A polarization measurement of the Crab would not only validate the instrument design (for future missions observing GRBs), but also improve our understanding of the emission mechanisms of the Crab emission. The flight plan included 8 hours of Crab observation but the flight was terminated before all data could be collected. The Crab was observed for only 1.8 hours. Background dominates the observations at flight altitudes, therefore a good background estimation is crucial for the analysis. The background depends on many flight and instrument parameters (altitude, instrument pointing, temperatures, etc). Estimating the Crab background using these parameters were a challenging task. 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 subsequently incorporated in our simulations and the instrument response which further improved our analysis. 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±0.24 and a normalization of 1.01±1.35 ph/keV/s/cm2 was measured. A polarization fraction of 0.43±0.4 and a polarization angle of 56◦±30◦ was measured for the phase integrated Crab observation in the 70-200 keV energy range. This was not sufficiently significant enough to further our understanding of the emission mechanism but was was statistically consistent with other experiments.
Wasti, Sambid Kumar, "Data Analysis for a Balloon Borne Compton Polarimeter" (2020). Doctoral Dissertations. 2557.