GEMS x-ray polarimeter performance simulations

The Gravity and Extreme Magnetism Small explorer (GEMS) is an X-ray polarization telescope designed as a NASA small explorer satellite mission. The X-ray Polarimeter on GEMS uses a Micropattern Time Projection Chamber to measure the polarization of astrophysical X-rays in the 2-10 keV band by imaging the track of the primary photoelectron generated by the incident X-ray and measuring its direction. We have simulated the expected sensitivity of the polarimeter to polarized X-rays. We use the simulation package Penelope to model the physics of the interaction of the initial photoelectron with the detector gas and to determine the distribution of charge deposited in the detector volume. We then model the charge diffusion in the detector, and produce simulated track images. Within the track reconstruction algorithm we apply cuts on the track shape and focus on the direction of the initial part of the photoelectron track in order to maximize the overall sensitivity of the instrument. Using this technique we have predicted instrument modulation factors mu(100) for 100% polarized X-rays ranging from 10% to over 60% across the 2-10 keV X-ray band. We also discuss the simulation program used to develop and model the algorithms used for triggering and measuring the energy of X-ray events in the polarimeter.