Abstract
X-ray polarimetry promises to give qualitatively new information bout high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested ahard X-ray polarimeter, X-<em>Calibur</em>, to be used in the focal plane of the InFOCuS grazing incidence hard X-ray telescope.X-<em>Calibur</em> combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 20−60 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation; in principal, a similar space-borne experiment could be operated in the 5−100 keV regime. X-<em>Calibur</em> achieves a high detection efficiency of order unity.
Highlights
Spectro-polarimetric X-ray observations are capable of providing additional information – namely the fraction and orientation of linear polarization – and would help to constrain different emission models [1, 2] of sources with compact emission regions and high X-ray fluxes such as mass-accreting black holes (BHs) and neutron stars
The source exhibits a polarization fraction of 20% at energies of 2.6 − 5.2 keV [3] and 46% ± 10% above 100 keV
The advantages of the X-Calibur/InFOCuS design are (i) a high detection efficiency by using more than 80% of photons impinging on the polarimeter, (ii) low background due to the usage of a focusing optics instead of large detector volumes, and (iii) minimization and control of systematic effects and achievement of a corresponding quantitative estimate thereof
Summary
The polarized synchrotron photons can be inverse-Compton (IC) scattered by relativistic electrons – weakening the fraction of polarization (but not erasing it) and imprinting a scattering angle dependence [8] to the observed fraction of polarization Such IC signals usually (but not always) appear in hard gamma-rays, where polarimetry is difficult, due to multiple scattering in pair production detectors. The advantages of the X-Calibur/InFOCuS design are (i) a high detection efficiency by using more than 80% of photons impinging on the polarimeter, (ii) low background due to the usage of a focusing optics instead of large detector volumes, and (iii) minimization and control of systematic effects and achievement of a corresponding quantitative estimate thereof. In order to measure the response to a polarized Xray beam we operated the X-Calibur polarimeter at the Cornell High Energy Synchrotron Source (CHESS) [13].
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