The Instrument of the Imaging X-Ray Polarimetry Explorer

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.

Calibration of the IXPE Focal Plane X-Ray Polarimeters to Polarized Radiation

“…A good knowledge of the position angle allows for comparing multiwavelength observations and detect secular variations. We recall that, at the level of the observatory, the knowledge of the position angle is 1 degree Soffitta et al (2021). Therefore, it is necessary to determine the polarization angle measurement accuracy with respect to the detector mechanical coordinates.…”

“…The Imaging X-ray Polarimetry Explorer (IXPE; Weisskopf et al 2016;Soffitta 2017;O'Dell et al 2019;Soffitta et al 2020;Ramsey et al 2021;Soffitta et al 2021;Weisskopf et al 2021) will be the first X-ray astronomy mission fully dedicated to polarimetry; it will expand our knowledge on X-ray sources, adding polarization data to temporal, spectral, and imaging ones allowing to obtain scientifically relevant measurements from several sources (e.g., neutron stars, black holes, active Galactic nuclei, supernova remnants, etc.). Despite the importance of polarimetric information, the only available statistically significant measurements of X-ray polarization were obtained for the Crab Nebula (Weisskopf et al 1976(Weisskopf et al , 1978 over 40 yr ago by using the crystal polarimeters aboard the Orbiting Solar Observatory 8 and recently by PolarLight (Feng et al 2020), albeit with a much smaller significance.…”

“…Gas Pixel Detector (GPD; Costa et al 2001;Bellazzini et al 2006Bellazzini et al , 2007Baldini et al 2021), and the Back-End Electronics (Barbanera et al 2021), which connects to the Detector Service Unit. The whole IXPE instrument has been built by INAF-IAPS and INFN, and it is described in more detail in Soffitta et al (2021).…”

Gamma-Ray Polarimetry of the Crab Pulsar Observed by POLAR

“…Additionally, as POLAR-2 will have an increased energy range, spanning from 30 keV to 800 keV, energy dependent polarization measurements will also be within the possibilities. Such measurements can be combined with those from photoelectron-track polarimeters onboard the IXPE mission (Soffitta et al 2021) and the eXTP mission (Zhang et al 2019b) in the near future. At the time of the launch of POLAR-2, IXPE is expected to have performed precise measurements of the Crab emission in the 2-10 keV energy range, while eXTP will likely increase the precision during, or after, the POLAR-2 mission.…”

Analysis of the data from photoelectric gas polarimeters

“…The response of a photoelectric gas polarimeter is the 2-dimensional image of photoelectron absorbed in its sensitive volume, projected on the plane of the detector (see Figure 1). We show in Figure 2 real photoelectron images generated from Xray photons absorbed in one of the flight Gas Pixel Detectors [4,5,6] built for the Imaging X-ray Polarimetry Explorer (IXPE, [10,11,12]). Absorbed photons had energy 2.3 keV and 6.4 keV for the events in Figure 2a and Figure 2b, respectively, which lie close to the boundaries of the instrument energy range, which is 2-8 keV.…”