The x-ray polarization of compact objects in x-ray binaries allows us to understand the complex spacetimes surrounding these sources. XL-Calibur is a state-of-the-art, balloon-borne telescope that measures the linear polarization of stellar-mass black holes, neutron stars, and nebulae in the 15-80 keV energy band. The selected energy range allows for observing coronal emission from black holes while also enabling us to narrow down on emission models from neutron stars, pulsars, and magnetars. Early in 2024, XL-Calibur will be launched from Kiruna, Sweden for approximately 10 days to observe Cyg X-1 and Cyg X-3, or other sources chosen based on flux levels at the time of flight. Observations might be coordinated with the recently launched Imaging x-ray Polarimetry Explorer mission which measures polarization in the complimentary 2-8 keV band. Combined XL-Calibur and IXPE observations will yield information on both soft and hard x-rays allowing us to decompose the total emission from black holes into thermal disk and coronal. We discuss the characterization of the XL-Calibur CdZnTe detectors, the telescope mirror and truss setup, and preliminary results from our most recent flight.
X-ray polarization measurements can provide unique information that is complementary to that obtained through spectroscopic or imaging observations. However, there have been few cases where significant x-ray polarization has been observed. XL-Calibur, conducted in collaboration between Japan, the United States of America, and Sweden, is a balloon-borne mission that aims to conduct high-sensitivity polarimetric observations in the hard x-ray band from 15 to 80 keV. The Japanese group is in charge of developing the Hard X-ray Telescope (HXT) with high light-gathering power. Optical adjustments were completed in 2020, and the performance of the HXT was measured in June 2021 at the SPring-8 (synchrotron radiation facility in Hyogo, Japan). Subsequently, in July 2022, the first observation was conducted from Sweden to Canada. After the flight, the HXT was recovered, and we measured its performance again. By comparing the HXT performances before and after the flight, we found no significant changes that can affect the second flight scheduled in 2024.
We have been developing silicon foil X-ray optics using a hot plastic deformation process for future astronomical observations. Our foil mirror is made of a 0.3-mm thick silicon wafer and is plastically deformed into a high-accurate conical shape with a curvature radius of ~100 mm. The angular resolution we evaluated using a test sample mirror was ~32 arcseconds in the best region. We have also successfully coated a platinum film on the foil mirror using the atomic layer deposition process. In this talk, we report on the fabrication method and the X-ray imaging capability of our silicon foil X-ray optics.
XL-Calibur is a balloon-borne mission for hard x-ray polarimetry. The first launch is currently scheduled from Sweden in summer 2022. Japanese collaborators provide a hard x-ray telescope to the mission. The telescope’s design is identical to the Hard X-ray Telescope (HXT, conically-approximated Wolter-I optics) on board ASTROH with the same focal length of 12 m and the aperture of 45 cm, which can focus x-rays up to 80 keV. The telescope is divided into three segments in the circumferential direction, and confocal 213 grazing-incidence mirrors are precisely placed in the primary and secondary sections of each segment. The surfaces of the mirrors are coated with Pt/C depth-graded multilayer to reflect hard x-rays efficiently by the Bragg reflection. To achieve the best focus, optical adjustment of all of the segments was performed at the SPring-8/BL20B2 synchrotron radiation facility during 2020. A final performance evaluation was conducted in June 2021 and the experiment yields the effective area of 175 cm2 and 73 cm2 at 30 keV and 50 keV, respectively, with its half-power diameter of the point spread function as 2.1 arcmin. The field of view, defined as the full width of the half-maximum of the vignetting curve, is 5.9 arcmin.
This paper introduces a second-generation balloon-borne hard X-ray polarimetry mission, XL-Calibur. X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as pulsars and binary black hole systems. The XL-Calibur contains a grazing incidence X-ray telescope with a focal plane detector unit that is sensitive to linear polarization. The telescope is very similar in design to the ASTRO-H HXT telescopes that has the world’s largest effective area above ~10 keV. The detector unit combines a low atomic number Compton scatterer with a CdZnTe detector assembly to measure the polarization making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. It also contains a CdZnTe imager at the bottom. The detector assembly is surrounded by the improved anti-coincidence shielding, giving a better sensitivity. The pointing system with arcsecond accuracy will be achieved.
We present the measurements of the spot size of a powerful X-ray generator at an ISAS 30m X-ray beamline facility. The generator was manufactured by Rigaku corporation. A parallel light is ideal to measure the point spread function of an X-ray telescope. However, we cannot make purely parallel light because we have to use an X-ray source with a finite size at a finite distance in a ground facility. When we evaluate a performance of X-ray telescope on the ground, an apparent size of an X-ray source often limits measurements of its point-spread function (PSF). The smaller X-ray source at the larger distance is needed to measure the PSF core of X-ray telescope. A high power X-ray source is also need to illuminate a telescope with a raster scan method that allows us precise measurements of a PSF tail. Therefore a powerful X-ray generator with a small spot size is strongly favoured in terms of the measurements of X-ray telescopes. We then introduced a powerful 18 kW rotating anode generator with a fine point focus at ISAS in 2016. The minimum and maximum voltage is 5 and 60 keV, respectively. A small spot size is technically more difficult on a lower voltage. In order to measure the spot size of the generator, we place a pinhole with a 0.1 mm diameter at 12.7 m away from the generator. The flipped image was taken at a CCD imager place at 33.5 m away from the generator. We made measurements of the Cu-K line at 8 keV with two options. One is a high-power mode (normal filament option) with a wider spot size whereas the other is a low-power mode with a finer size (fine filament option). As a result, we found that a spot size of the normal and fine filament options are 2.8 mm and 0.80 mm (FWZI), respectively. We supplied a bias voltage of 800 V for the fine filament option. The spot size of 0.8 mm is equivalent to the angular extent of as good as 6 arcsecs in the ISAS X-ray beamline facility.
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