Measuring the Polarization of Hard X-Ray Solar Flares with GRAPE

The degree of linear polarization of hard X-rays from solar flares can provide a better understanding of the particle acceleration mechanisms through a determination of the particle acceleration geometry. Because of difficulties in measuring the polarization, past experiments have been unable to put many constraints on solar flare models. The Gamma RAy Polarimetry Experiment (GRAPE) has been designed to measure polarization in the 50 - 500 keV energy range, thus minimizing the thermal contamination often affecting other X-ray flare polarization experiments. On September 23-24, 2011, GRAPE was flown on a high altitude balloon from the NASA scientific ballooning facility in Fort Sumner, NM. Over the course of the 26-hour flight, whose primary purpose was the measurement of polarization from the Crab Nebula, several large flares took place from active region 1302. This included two flares of class M2.8 and M3.1 during the solar observation period, when the active region was about 30° from the eastern limb. The configuration used in the Fort Sumner flight was intended to make measurements of steady state sources adding a degree of difficulty to the analysis of transient sources. To assist with the data analysis, a mass model of the full flight payload has been developed using GEANT4. This paper focuses on the analysis of the flight data and the search for hard X-ray polarization from these two flares. Although there is no detection of polarization, our analysis places constraints on the level of polarization. We discuss these results in the context of future observations of solar flares by the GRAPE instrument.