Performance verification of the Gravity and Extreme Magnetism Small explorer (GEMS) x-ray polarimeter

Abstract

Polarimetry is a powerful tool for astrophysical observations that has yet to be exploited in the X-ray band. For satellite-borne and sounding rocket experiments, we have developed a photoelectric gas polarimeter to measure X-ray polarization in the 2-10 keV range utilizing a time projection chamber (TPC) and advanced micro-pattern gas electron multiplier (GEM) techniques. We carried out performance verification of a flight equivalent unit (1/4 model) which was planned to be launched on the NASA Gravity and Extreme Magnetism Small Explorer (GEMS) satellite. The test was performed at Brookhaven National Laboratory, National Synchrotron Light Source (NSLS) facility in April 2013. The polarimeter was irradiated with linearly-polarized monochromatic X-rays between 2.3 and 10.0 keV and scanned with a collimated beam at 5 different detector positions. After a systematic investigation of the detector response, a modulation factor ≥35% above 4 keV was obtained with the expected polarization angle. At energies below 4 keV where the photoelectron track becomes short, diffusion in the region between the GEM and readout strips leaves an asymmetric photoelectron image. A correction method retrieves an expected modulation angle, and the expected modulation factor, ∼20% at 2.7 keV. Folding the measured values of modulation through an instrument model gives sensitivity, parameterized by minimum detectable polarization (MDP), nearly identical to that assumed at the preliminary design review (PDR).

Performance of the PRAXyS X-ray polarimeter

“…To maximize sensitivity we have developed a gas polarimeter that employs the Time Projection Chamber (TPC) technique [8,9,10,11,12]. In this case the detection plane is parallel to the incident X-rays.…”

Development of the GEM-TPC X-ray Polarimeter with the Scalable Readout System

“…The Bragg peak cut improves the modulation factor of high-energy (> 5 keV) X-rays by approximately 10%. A preliminary result of the modulation factor for 6.4 keV X-rays at 7.6 mm drift length, which is near the optical axis of the polarimeter combined with an X-ray mirror, is approximately 50%, comparable to that of the flight equivalent polarimeter [12].…”

“…Figure 3 shows 6.4 keV spectra at various detector positions. The energy resolution (FWHM) of all the spectra is 22%, which is larger than that of a prototype of the flight polarimeter, 16% [12]. The broader energy resolution is due to a non-uniform gain of the GEM foil [13] and could be improved by replacing it with a foil of improved quality.…”

“…An electron attachment coefficient at the drift field of 374 V cm −1 is ∼ 0.05 cm −1 , derived by exponential fitting to the spectral peak as a function of the drift distance. The spectral peak shift due to electron attachment (not seen in the flight equivalent polarimeter [12]), can be reduced by long-term bake-out, which was performed, for the polarimeter chamber, with the temperature of 80 • C for a week before the beam test. Figure 4 shows photoelectron tracks with various energies at different X-ray irradiation positions.…”

Instrumentation and Future Missions in the Upcoming Era of X-ray Polarimetry

“…The TPC design was proposed on board GEMS [68][69][70][71][72] and PRAXyS [73][74][75][76] mission proposals. The GPD design was proposed in the past in many mission proposals (the most relevant are: IXO [77], NHXM [78], POLARIX [79], ESA Medium class mission XIPE [80]) and currently in the IXPE mission [81,82] approved by NASA and eXTP mission [83] selected as background mission 1 by the Chinese Academy of Sciences.…”