The XL-Calibur balloon-borne hard X-ray polarimetry mission comprises a Compton-scattering polarimeter placed at the focal point of an X-ray mirror. The polarimeter is housed within a BGO anticoincidence shield, which is needed to mitigate the considerable background radiation present at the observation altitude of ∼40 km. This paper details the design, construction and testing of the anticoincidence shield, as well as the performance measured during the week-long maiden flight from Esrange Space Centre to the Canadian Northwest Territories in July 2022. The in-flight performance of the shield followed design expectations, with a veto threshold <100 keV and a measured background rate of ∼0.5 Hz (20–40 keV). This is compatible with the scientific goals of the mission, where %-level minimum detectable polarisation is sought for a Hz-level source rate.

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 ASTRO-H 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 cm(2) and 73 cm(2) 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.

XL-Calibur is a balloon-borne hard X-ray polarimetry mission, the first flight of which is currently foreseen for 2022. XL-Calibur carries an X-ray telescope consists of 213 Wolter I grazing-incidence mirrors which are nested in a coaxial and cofocal configuration. The optics design is nearly identical to the Hard X-ray Telescope (HXT) on board the ASTRO-H satellite. The telescope was originally fabricated for the Formation Flying Astronomical Survey Telescope (FFAST) project. However, the telescope can be used for XL-Calibur, since the FFAST project was terminated before completion. The mirror surfaces are coated with Pt/C depth-graded multilayers to reflect hard X-rays above 10 keV by Bragg reflection. The effective area of the telescope is larger than 300 cm(2) at 20 keV. This paper reports the current status of the telescope for XL-Calibur.

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 tdaydays, the Minimum Detectable Polarisation (at 99% confidence level) is similar to 2%.t(day)(-1/2). The energy resolution at 40 keV is-5.9 keV. The aim of this paper is to describe the design and performance of the XL-Caliburmission, as well as the foreseen science programme.

This paper introduces a second-generation balloon-borne hard X-ray polarimetry mission, XL-Calibur.(1) The XL-Calibur will follow up on the X-Calibur mission which was flown from Dec. 29, 2018 for a 2.5 days balloon flight from McMurdo (the Antarctic). 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. XL-Calibur will use the same type of mirror. 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 a BGO anticoincidence shield. The pointing system with arcsecond accuracy will be achieved by the WASP (Wallops Arc Second Pointer) from NASA's Wallops Flight Facility. A first flight of the XL-Calibur is currently foreseen for 2022, flying from Sweden.

In recent years, a number of purpose-built scintillator-based polarimeters have studied bright astronomical sources for the first time in the hard X-ray band (tens to hundreds of keV). The addition of polarimetry can help data interpretation by resolving model-dependent degeneracies. The typical instrument approach is that incident X-rays scatter off a plastic scintillator into an adjacent scintillator cell. In all missions to date, the scintillators are read out using traditional vacuum tube photo-multipliers (PMTs). The advent of solid-state PMTs (“silicon PM” or “MPPC”) is attractive for space-based instruments since the devices are compact, robust and require a low bias voltage. We have characterised the plastic scintillator, EJ-248M, optically coupled to a multi-pixel photon counter (MPPC) and read out with the Citiroc ASIC. A light-yield of 1.6 photoelectrons/keV has been obtained, with a low energy detection threshold of ≲5 keV at room temperature. We have also constructed an MPPC-based polarimeter-demonstrator in order to investigate the feasibility of such an approach for future instruments. Incident X-rays scatter from a plastic-scintillator bar to surrounding cerium-doped GAGG (Gadolinium Aluminium Gallium Garnet) scintillators yielding time-coincident signals in the scintillators. We have determined the polarimetric response of this set-up using both unpolarised and polarised ∼50 keV X-rays. We observe a clear asymmetry in the GAGG counting rates for the polarised beam. The low-energy detection threshold in the plastic scintillator can be further reduced using a coincidence technique. The demonstrated polarimeter design shows promise as a space-based Compton polarimeter and we discuss ways in which our polarimeter can be adapted for such a mission.

Gamma-ray bursts (GRBs) are exceptionally bright electromagnetic events occurring daily on the sky. The prompt emission is dominated by X-/gamma-rays. Since their discovery over 50 years ago, GRBs are primarily studied through spectral and temporal measurements. The properties of the emission jets and underlying processes are not well understood. A promising way forward is the development of missions capable of characterising the linear polarisation of the high-energy emission. For this reason, the SPHiNX mission has been developed for a small-satellite platform. The polarisation properties of incident high-energy radiation (50-600 keV) are determined by reconstructing Compton scattering interactions in a segmented array of plastic and Gd3Al2Ga3O12(Ce) (GAGG(Ce)) scintillators. During a two-year mission, similar to 200 GRBs will be observed, with similar to 50 yielding measurements where the polarisation fraction is determined with a relative error <= 10%. This is a significant improvement compared to contemporary missions. This performance, combined with the ability to reconstruct GRB localisation and spectral properties, will allow discrimination between leading classes of emission models.

Black hole binary (BHB) systems comprise a stellar-mass black hole and a closely orbiting companion star. Matter is transferred from the companion to the black hole, forming an accretion disk, corona and jet structures. The resulting release of gravitational energy leads to the emission of X-rays¹. The radiation is affected by special/general relativistic effects, and can serve as a probe for the properties of the black hole and surrounding environment, if the accretion geometry is properly identified. Two competing models describe the disk–corona geometry for the hard spectral state of BHBs, based on spectral and timing measurements^2,3. Measuring the polarization of hard X-rays reflected from the disk allows the geometry to be determined. The extent of the corona differs between the two models, affecting the strength of the relativistic effects (such as enhancement of the polarization fraction and rotation of the polarization angle). Here, we report observational results on the linear polarization of hard X-ray emission (19–181 keV) from a BHB, Cygnus X-1⁴, in the hard state. The low polarization fraction, <8.6% (upper limit at a 90% confidence level), and the alignment of the polarization angle with the jet axis show that the dominant emission is not influenced by strong gravity. When considered together with existing spectral and timing data, our result reveals that the accretion corona is either an extended structure, or is located far from the black hole in the hard state of Cygnus X-1.

This Article contains a typographical error in the legend of Figure 2. "Gaussian 1, 2 and 3& #x1D70E;" should read: "Gaussian 1, 2 and 3σ". 

Polarimetric observations of celestial sources in the hard X-ray band stand to provide new information on emission mechanisms and source geometries. PoGO+ is a Compton scattering polarimeter (20-150 keV) optimised for the observation of the Crab (pulsar and wind nebula) and Cygnus X-1 (black hole binary), from a stratospheric balloon-borne platform launched from the Esrange Space Centre in summer 2016. Prior to flight, the response of the polarimeter has been studied with polarised and unpolarised X-rays allowing a Geant4-based simulation model to be validated. The expected modulation factor for Crab observations is found to be M-Crab = (41.75 +/- 0.85)%, resulting in an expected Minimum Detectable Polarisation (MDP) of 7.3% for a 7 day flight. This will allow a measurement of the Crab polarisation parameters with at least 5 sigma statistical significance assuming a polarisation fraction similar to 20% - a significant improvement over the PoGOLite Pathfinder mission which flew in 2013 and from which the PoGO+ design is developed.