Fermi Gamma-ray Space Telescope

A PCA Approach to Estimating the Background for the GRAPE Balloon Experiment

S. K. Wasti
P. F. Bloser, J. S. Legere, M. L. McConnell, J. M. Ryan

Abstract:

Estimating the gamma ray background of a balloon borne experiment has always been a challenge. The background typically depends on many variables including altitude and payload orientation. Background may also depend on instrument parameters like temperatures. Estimating the background is especially difficult when one or more of these parameters change significantly during the flight, which is often the case for a balloon borne experiment. Identifying the important parameters will facilitate the background analysis. The Gamma Ray Polarimeter Experiment (GRAPE), a balloon borne polarimeter for 50~300 keV gamma rays, successfully flew in 2011 and 2014. The main goal of these balloon flights was to measure the gamma ray polarization of the Crab Nebula. Analysis of data from the first two balloon flights of GRAPE has been challenging due to significant changes in the background level during each flight. We have developed a technique based on the Principle Component Analysis (PCA) to estimate the background for the Crab observation. We found that the background depended mostly on the atmospheric depth, pointing zenith angle and instrument temperatures. Incorporating Anti-coincidence shield data (which served as a surrogate for the background) was also found to improve the analysis. Here, we describe the analysis and present results from the 2014 balloon flight.