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.
The new generation of x-ray and gamma-ray detectors employ cryogenic detectors known as transition-edge sensors (TES) due to their high energy resolution and photon detection rates. These detectors require a refrigeration module that can operate at the transition temperature of the TES’s superconducting film—usually at mK temperatures. DR-TES consists of a novel mini-dilution refrigerator (DR) from Chase Research Cryogenics that can be used in balloon-borne missions to cool detectors to temperatures between 10 to 100mK. To test the viability of this DR module, we will be cooling down a SLEDGEHAMMER detector fabricated by the National Institute of Standards and Technology quantum sensor group. The SLEDGEHAMMER microcalorimeter uses TESs coupled to superconducting quantum interference devices which are in turn coupled to microwave resonators to detect x-rays and gamma-rays. We plan to fly the SLEDGEHAMMER detector cooled by the mini-DR on a stratospheric balloon flight in August of 2024 at Fort Sumner, NM. As a follow-up mission, 511-CAM will use a modified version of the detector to map the 511keV emission from the galactic center region.
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.
The 511 keV γ-ray emission from the galactic center region may fully or partially originate from the annihilation of positrons from dark matter particles with electrons from the interstellar medium. Alternatively, the positrons could be created by astrophysical sources, involving exclusively standard model physics. We describe here a new concept for a 511 keV mission called 511-CAM (511 keV gamma-ray camera using microcalorimeters) that combines focusing γ-ray optics with a stack of transition edge sensor microcalorimeter arrays in the focal plane. The 511-CAM detector assembly has a projected 511 keV energy resolution of 390 eV full width half maximum or better, and improves by a factor of at least 11 on the performance of state-of-the-art Ge-based Compton telescopes. Combining this unprecedented energy resolution with sub-arcmin angular resolutions afforded by Laue lens or channeling optics could make substantial contributions toward identifying the origin of the 511 keV emission through discovering and characterizing point sources and measuring line-of-sight velocities of the emitting plasmas.
Core collapse supernovae are thought to be one of the main sources in the galaxy of elements heavier than iron. Understanding the origin of the elements is thus tightly linked to our understanding of the explosion mechanism of supernovae and supernova nucleosynthesis. X-ray and gamma-ray observations of young supernova remnants, combined with improved theoretical modeling, have resulted in enormous improvements in our knowledge of these events. The isotope Ti44 is one of the most sensitive probes of the innermost regions of the core collapse engine, and its spatial and velocity distribution are key observables. Hard x-ray imaging spectroscopy with the Nuclear Spectroscopic Telescope Array (NuSTAR) has provided new insights into the structure of the supernova remnant Cassiopeia A (Cas A), establishing the convective nature of the supernova engine. However, many questions about the details of this engine remain. We present here the concept for a balloon-borne follow-up mission called A SuperConducting ENergetic x-ray Telescope (ASCENT). ASCENT uses transition edge sensor gamma-ray microcalorimeter detectors with a demonstrated 55-eV full-width half maximum energy resolution at 97 keV. This 8- to 16-fold improvement in energy resolution over NuSTAR will allow for high-resolution imaging and spectroscopy of the Ti44 emission. This will allow for a detailed reconstruction of gamma-ray line redshifts, widths, and shapes, allowing us to address questions such as, What is the source of the neutron star kicks? What is the dominant production pathway for Ti44? Is the engine of Cas A unique?
NIST has developed microwave multiplexed microcalorimeter arrays for the detection of hard X-rays andγ-rays (Bennett et al. 2012, Mates et al. 2017). The arrays are made of tin or bismuth absorbers that are read-out with arrays of Transition Edge Sensors (TES). Each TES is coupled via a SQUID to a microwave resonator, and a single microwave line is used to sample the response of the resonators of all pixels. The detector arrays achieve an energy resolution of 55 eV FWHM at 97 keV. We report here on the performance of a 34-pixels prototype TES with a collimated 50μm diameter 20-50 keV X-ray beam as well as a Eu(152) source for a future balloon flight. We will furthermore describe a planned stratospheric balloon flight that will be used to demonstrate the performance of a novel mini-dilution refrigerator and the 34-pixels prototype detector in a space environment.
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.
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