X-ray polarimetry promises to deliver qualitatively new insights into the astrophysics of black hole binaries,
neutron stars, gamma ray bursts, and supermassive black holes and their jets.
It also presents unique avenues to probe general relativity and to test Lorentz Invariance with unprecedented sensitivity.
The Washington University group has developed a uniquely sensitive hard X-ray polarimeter for the focal plane of a focussing hard X-ray telescope.
X-Calibur measures polarization based on detecting the scattering angle of Compton scattered X-ray photons. The photons scatter preferentially into a direction perpendicular to the orientation of the electric field vector.
The experiment uses a low-atomic-number (plastic) slab as scatterer and
high-atomic-number solid state detector ("CZT" detectors) to absorb the scattered
It operates in the 2-80 keV range when used on a satellite and in the 20-80 keV range when used on a balloon.
The instrument combines a >80% detection efficiency, with a Full Width Half Maximum (FWHM) energy resolution of 5 keV, and a very low background level.
X-Calibur is the only hard X-ray polarimeter which achieves a sensitivity close to the limiting sensitivity dictated by the physics of Compton interactions.
In Fall 2014, The experiment will be flown in the focal plane of the InFOCuS X-ray telescope
on a one-day balloon flight from Fort Sumner (NM) and on a >20 day long duration balloon flight from McMurdo (antarctic).
The InFOCuS telescope will be contributed by Dr. Scott Barthelmy and coworkers
from the Goddard Space Flight Center.
The telescope makes use of an 8m-focal-length X-ray mirror developed by Dr. Hideyo Kunieda.
The one-day flight will allow us to make sensitive spectropolarimetric observations of five galactic sources and one extragalactic blazar with a minimum detectable polarization degree of 4% for bright sources with a Crab-like flux. The results will contribute to our understanding of binary black hole systems, pulsars and accreting neutron stars. Furthermore, the flight data will allow us to study the systematics limiting hard X-ray polarimeters.
For more information about X-Calibur, you can read an
article in the Record
(Washington University's newspaper)
listen to a podcast.