The polarization of light provides two observables, polarization degree and angle, that are very sensitive to the anisotropies of the emission region; therefore, polarization, combined with spectral and timing analysis, can be a powerful tool to study the geometry of astronomical sources. Regular astrophysical observations of X-ray polarization will soon become a reality, as several observatories with an X-ray polarimeter on board are now being developed. The NASA SMEX mission
IXPE (Weisskopf et al. 2016), in the 1–10 keV energy range, and the Indian
POLIX (5–30 keV), are both scheduled to launch in 2021, as well as the rocket-based
REDSox (0.2–0.8 keV) (Gaenther et al. 2017), while the balloon-borne
X-Calibur (Beilicke et al. 2014) and
PoGO+ (Chauvin et al. 2018) are already flying. Additionally, the Chinese–European
eXTP (Zhang et al. 2016) (1– 10 keV), is scheduled for launch in 2025, while the narrow band (250 eV)
LAMP (She et al. 2015) and the broad band (0.2–60 keV)
XPP (Krawczynski et al. 2019; Jahoda et al. 2019) are still at the concept stage. Many of these upcoming polarimeters employ the gas pixel detector technology, or GPD, which was recently put to test by the CubeSat
PolarLight (Feng et al. 2019), a small polarimeter without optics, that was able to measure the polarization of the Crab nebula (Feng et al. 2020).
I am a member of the science teams of IXPE and eXTP. Here you can find some of our white papers: