Abstract
Due to be launched in late 2021, the Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Small Explorer mission designed to perform polarization measurements in the 2–8 keV band, complemented with imaging, spectroscopy and timing capabilities. At the heart of the focal plane is a set of three polarization-sensitive Gas Pixel Detectors (GPD), each based on a custom ASIC acting as a charge-collecting anode.In this paper we shall review the design, manufacturing, and test of the IXPE focal-plane detectors, with particular emphasis on the connection between the science drivers, the performance metrics and the operational aspects. We shall present a thorough characterization of the GPDs in terms of effective noise, trigger efficiency, dead time, uniformity of response, and spectral and polarimetric performance. In addition, we shall discuss in detail a number of instrumental effects that are relevant for high-level science analysis—particularly as far as the response to unpolarized radiation and the stability in time are concerned.
Highlights
Due to the limited sensitivity achievable with conventional techniques, polarimetry of X-ray astrophysical sources is, to date, essentially limited to high-significance detections for a single bright source, the Crab Nebula [1,2]
At the heart of the focal plane is a set of three polarization-sensitive Gas Pixel Detectors (GPD), each based on a custom Application Specific Integrated Circuit (ASIC) acting as a charge-collecting anode
Compared with other Gas Electron Multiplier (GEM) devices customarily used in high-energy physics applications, the main peculiarity of those developed for the Imaging X-ray Polarimetry Explorer (IXPE) mission is their fine pitch, which is in turn dictated by the necessity to preserve as much as possible the morphology of the photoelectron track and match the sampling capabilities of the readout plane
Summary
Due to the limited sensitivity achievable with conventional techniques, polarimetry of X-ray astrophysical sources is, to date, essentially limited to high-significance detections for a single bright source, the Crab Nebula [1,2]. Gas Pixel Detectors (GPD) [3] were proposed in the early 2000 as the first practical implementation of soft X-ray photoelectric polarimetry, with the potential for a leap in sensitivity by more than an order of magnitude.1 This technology opened the way to mission concepts offering for the first time the opportunity to observe tens of sources for precision measurements of their polarimetric properties, providing invaluable insight into their geometries and the physical processes at play. Having accumulated the equivalent of ∼25 GPD-years of test data at the time of writing (see Fig. 1), we uncovered some more subtle instrumental effects that are understood and must be taken into account with dedicated calibrations during the IXPE mission Minimizing these effects in an improved GPD design offers an exciting research opportunities for the generation of astrophysical polarimeters
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