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Register a new userDesign and tests of the hard X-ray polarimeter X-Calibur
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X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCuS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10-80 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity.
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X-ray polarimetry promises to give new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCuS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10-80 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity.
X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, neutron stars, and gamma-ray bursts. We designed, built and tested a X-ray polarimeter, X-Calibur, to be used in the focal plane of the balloon-borne InFOCuS grazing incidence X-ray telescope. X-Calibur combines a low-Z scatterer with a CZT detector assembly to measure the polarization of 20-80keV X-rays making use of the fact that polarized photons scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of ~80%. The X-Calibur detector assembly is completed, tested, and fully calibrated. The response to a polarized X-ray beam was measured successfully at the Cornell High Energy Synchrotron Source. This paper describes the design, calibration and performance of the X-Calibur polarimeter. In principle, a similar space-borne scattering polarimeter could operate over the broader 2-100keV energy band.
X-ray polarimetry has seen a growing interest in recent years. Improvements in detector technology and focusing X-ray optics now enable sensitive astrophysical X-ray polarization measurements. These measurements will provide new insights into the processes at work in accreting black holes, the emission of X-rays from neutron stars and magnetars, and the structure of AGN jets. X-Calibur is a balloon-borne hard X-ray scattering polarimeter. An X-ray mirror with a focal length of 8 m focuses X-rays onto the detector, which consists of a plastic scattering element surrounded by Cadmium-Zinc-Telluride detectors, which absorb and record the scattered X-rays. Since X-rays preferentially scatter perpendicular to their polarization direction, the polarization properties of an X-ray beam can be inferred from the azimuthal distribution of scattered X-rays. A close alignment of the X-ray focal spot with the center of the detector is required in order to reduce systematic uncertainties and to maintain a high photon detection efficiency. This places stringent requirements on the mechanical and thermal stability of the telescope structure. During the flight on a stratospheric balloon, X-Calibur makes use of the Wallops Arc-Second Pointer (WASP) to point the telescope at astrophysical sources. In this paper, we describe the design, construction, and test of the telescope structure, as well as its performance during a 25-hour flight from Ft. Sumner, New Mexico. The carbon fiber-aluminum composite structure met the requirements set by X-Calibur and its design can easily be adapted for other types of experiments, such as X-ray imaging or spectroscopic telescopes.
In the 50 years since the advent of X-ray astronomy there have been many scientific advances due to the development of new experimental techniques for detecting and characterising X-rays. Observations of X-ray polarisation have, however, not undergone a similar development. This is a shortcoming since a plethora of open questions related to the nature of X-ray sources could be resolved through measurements of the linear polarisation of emitted X-rays. The PoGOLite Pathfinder is a balloon-borne hard X-ray polarimeter operating in the 25 - 240 keV energy band from a stabilised observation platform. Polarisation is determined using coincident energy deposits in a segmented array of plastic scintillators surrounded by a BGO anticoincidence system and a polyethylene neutron shield. The PoGOLite Pathfinder was launched from the SSC Esrange Space Centre in July 2013. A near-circumpolar flight was achieved with a duration of approximately two weeks. The flight performance of the Pathfinder design is discussed for the three Crab observations conducted. The signal-to-background ratio for the observations is shown to be 0.25$pm$0.03 and the Minimum Detectable Polarisation (99% C.L.) is (28.4$pm$2.2)%. A strategy for the continuation of the PoGOLite programme is outlined based on experience gained during the 2013 maiden flight.
XL-Calibur is a hard X-ray (15-80 keV) polarimetry mission operating from a stabilised balloon-borne platform in the stratosphere. It builds on heritage from the X-Calibur mission, which observed the accreting neutron star GX 301-2 from Antarctica, between December 29th 2018 and January 1st 2019. The XL-Calibur design incorporates an X-ray mirror, which focusses X-rays onto a polarimeter comprising a beryllium rod surrounded by Cadmium Zinc Telluride (CZT) detectors. The polarimeter is housed in an anticoincidence shield to mitigate background from particles present in the stratosphere. The mirror and polarimeter-shield assembly are mounted at opposite ends of a 12 m long lightweight truss, which is pointed with arcsecond precision by WASP - the Wallops Arc Second Pointer. The XL-Calibur mission will achieve a substantially improved sensitivity over X-Calibur by using a larger effective area X-ray mirror, reducing background through thinner CZT detectors, and improved anticoincidence shielding. When observing a 1 Crab source for $t_{rm day}$ days, the Minimum Detectable Polarisation (at 99% confidence level) is $sim$2$%cdot t_{rm day}^{-1/2}$. The energy resolution at 40 keV is $sim$5.9 keV. The aim of this paper is to describe the design and performance of the XL-Calibur mission, as well as the foreseen science programme.
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