XL-Calibur observes black holes such as Cygnus X1, and measures for the first time the polarization of the X-ray emission. The polarization makes it possible to test models of the origin of the X-rays and to map out the spacetime surrounding the black hole. The image shows an artist’s impression of a black hole (left) swallowing matter from a companion star (right). Image credit: NASA, CXC (Weisskopf et al.).

XL-Calibur has the following science goals:

  • Observations of the hard X-ray emission from stellar-mass black holes in X-ray binaries, such as Cyg X−1 and GX 339−4, will constrain the properties of the X-ray bright coronas. Together with IXPE, our results will help disentangle the polarisation of the thermal accretion disk emission, and the direct and reflected coronal emission.
  • Precision measurements of the birefringent properties of the QED vacuum surrounding highly-magnetised accreting pulsars, like Her X−1, GX 301−2, and Vela X−1, will be particularly informative at the energies of their Cyclotron Resonant Scattering Features (CRSF), where the competition of QED and plasma birefringence is expected to lead to extremely high and strongly energy-dependent polarisation. Together with those from IXPE, XL-Calibur observations have the potential to determine the emission geometry and to study the impact of strong QED effects on the birefringence of the magnetised plasma and the polarisation-dependent scattering cross-sections.
  • XL-Calibur can distinguish between competing emission models of the rotation-powered Crab pulsar, an archetypical cosmic particle accelerator. XL-Calibur’s high sensitivity will allow phase-resolved polarimetry, cleanly separating contributions from the pulsar and from the nebula.

More information can be found here and here.