Abstract
Abstract While X-ray spectroscopy, timing, and imaging have improved much since 1962 when the first astronomical nonsolar source was discovered, especially wi the launch of the Newton/X-ray Multi-Mirror Mission, Rossi/X-ray Timing Explorer, and Chandra/Advanced X-ray Astrophysics Facility, the progress of X-ray polarimetry has been meager. This is in part due to the lack of sensitive polarization detectors, which in turn is a result of the fate of approved missions and because celestial X-ray sources appear less polarized than expected. Only one positive measurement has been available until now: the Orbiting Solar Observatory measured the polarization of the Crab Nebula in the 1970s. The advent of microelectronics techniques has allowed for designing a detector based on the photoelectric effect of gas in an energy range where the optics are efficient at focusing in X-rays. Here we describe the instrument, which is the major contribution of the Italian collaboration to the Small Explorer mission called IXPE, the Imaging X-ray Polarimetry Explorer, which will launch in late 2021. The instrument is composed of three detector units based on this technique and a detector service unit. Three mirror modules provided by Marshall Space Flight Center focus X-rays onto the detectors. We show the technological choices, their scientific motivation, and results from the calibration of the instrument. IXPE will perform imaging, timing, and energy-resolved polarimetry in the 2–8 keV energy band opening this window of X-ray astronomy to tens of celestial sources of almost all classes.
Highlights
Since the dawn of X-ray Astronomy, polarimetry was considered a ’holy grail’ due to the characteristics of the radiation emitted by the celestial sources and their, typically, non-spherical geometry
Due to the fact that the beryllium window of the detector and the supporting titanium frame is at high potential positive ions may interact with the top Gas Pixel Detector (GPD) structure producing eventual secondary photons and a possible failure of the high voltage system
The GPD, located in the GPD housing, is a gas detector for which the charge-signal is readout by a matrix of 105k pixels (300 ×352 pixels arranged in a 50 μm pitch hexagonal pattern) of a dedicated, custom Complementary Metal-Oxide Semiconductor (CMOS) ApplicationSpecific Integrated Circuit (ASIC) (Bellazzini et al 2006)
Summary
Since the dawn of X-ray Astronomy, polarimetry was considered a ’holy grail’ due to the characteristics of the radiation emitted by the celestial sources and their, typically, non-spherical geometry. IXPE (Weisskopf et al 2008, 2016), see Figure 1, is a SMEX scientific mission selected by NASA in January 2017 with a large contribution of Agenzia Spaziale Italiana (ASI) to be launched in 2021 aboard a SpaceX Falcon-9 rocket It will measure polarization in X-rays from neutron stars, from stellar-mass black holes and from Active Galactic Nuclei (AGN). Modern X-ray telescope missions are designed for staring or dithering observation Each detector is mounted, with respect to the one, on the top-deck of the spacecraft with an angle, around the Z-axis, (the axis of the incoming photon beam, see Figure 1) of 120◦ Such disposition allow for a sensitive reduction and check of spurious effects
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