XL-Calibur is a balloon-borne Compton polarimeter for X-rays in the ∼15–80 keV range. Using an X-ray mirror with a 12 m focal length for collecting photons onto a beryllium scattering rod surrounded by CZT detectors, a minimum-detectable polarization as low as ∼3% is expected during a 24-hour on-target observation of a 1 Crab source at 45° elevation. Systematic effects alter the reconstructed polarization as the mirror focal spot moves across the beryllium scatterer, due to pointing offsets, mechanical misalignment or deformation of the carbon-fiber truss supporting the mirror and the polarimeter. Unaddressed, this can give rise to a spurious polarization signal for an unpolarized flux, or a change in reconstructed polarization fraction and angle for a polarized flux. Using bench-marked Monte-Carlo simulations and an accurate mirror point-spread function characterized at synchrotron beam-lines, systematic effects are quantified, and mitigation strategies discussed. By recalculating the scattering site for a shifted beam, systematic errors can be reduced from several tens of percent to the few-percent level for any shift within the scattering element. The treatment of these systematic effects will be important for any polarimetric instrument where a focused X-ray beam is impinging on a scattering element surrounded by counting detectors.

IGR J16320-4751 is a highly obscured HMXB source containing a very slow neutron star (P-spin similar to 1300 sec) orbiting its supergiant companion star with a period of similar to 9 days. It shows high column density (N-H similar to 2 - 5 x 10(23) cm(-2)) in the spectrum, and a large variation in flux along the orbit despite not being an eclipsing source. We report on some peculiar timing and spectral features from archival XMM-Newton observation of this source including 8 observations taken during a single orbit. The pulsar shows large timing variability in terms of average count rate from different observations, flaring activity, sudden changes in count rate, cessation of pulsation, and variable pulse profile even from observations taken a few days apart. We note that IGR J16320-4751 is among a small number of sources for which this temporary cessation of pulsation in the light curve has been observed. A time-resolved spectral analysis around the segment of missing pulse shows that variable absorption is deriving such a behavior in this source. Energy resolved pulse profiles in 6.2-6.6 keV band which has a partial contribution from Fe K-alpha photons, show strong pulsation. However, a more systematic analysis reveals a flat pulse profile from the contribution of Fe K-alpha photons in this band implying a symmetric distribution for the material responsible for this emission. Soft excess emission below 3 keV is seen in 6 out of 11 spectra of XMM-Newton observations.

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.

X-Calibur is a balloon-borne telescope that measures the polarization of high-energy X-rays in the 15-50 keV energy range. The instrument makes use of the fact that X-rays scatter preferentially perpendicular to the polarization direction. A beryllium scattering element surrounded by pixellated CZT detectors is located at the focal point of the InFOC mu S hard X-ray mirror. The instrument was launched for a long-duration balloon (LDB) flight from McMurdo (Antarctica) on December 29, 2018, and obtained the first constraints of the hard X-ray polarization of an accretion-powered pulsar. Here, we describe the characterization and calibration of the instrument on the ground and its performance during the flight, as well as simulations of particle backgrounds and a comparison to measured rates. The pointing system and polarimeter achieved the excellent projected performance. The energy detection threshold for the anticoincidence system was found to be higher than expected and it exhibited unanticipated dead time. Both issues will be remedied for future flights. Overall, the mission performance was nominal, and results will inform the design of the follow-up mission XL-Calibur, which is scheduled to be launched in summer 2022.

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.

We report the results from a detailed timing and spectral study of a transient X-ray pulsar, 4U 1901+03 during its 2019 outburst. We performed broadband spectroscopy in the 1-70 keV energy band using four observations made with Swift and NuSTAR at different intensity levels. Our timing results reveal the presence of highly variable pulse profiles dependent on both luminosity and energy. Our spectroscopy results showed the presence of a cyclotron resonance scattering feature (CRSF) at similar to 30 keV. This feature at 30 keV is highly luminosity and pulse phase dependent. Phase-averaged spectra during the last two observations, made close to the declining phase of the outburst, showed the presence of this feature at around 30 keV. The existence of CRSF at 30 keV during these observations is well supported by an abrupt change in the shape of pulse profiles found close to this energy. We also found that 30 keV feature was significantly detected in the pulse phase-resolved spectra of observations made at relatively high luminosities. Moreover, all spectral fit parameters showed a strong pulse phase dependence. In line with the previous findings, an absorption feature at around 10 keV is significantly observed in the phase-averaged X-ray spectra of all observations and also showed a strong pulse phase dependence.

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.

The accretion-powered X-ray pulsar GX 301-2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late 2018 December, with contiguous observations by the Neutron star Interior Composition Explorer Mission (NICER) X-ray telescope, the Swift X-ray Telescope and Burst Alert Telescope, and the Fermi Gamma-ray Burst Monitor spanning several months. The observations detected the pulsar in a rare apastron flaring state coinciding with a significant spin up of the pulsar discovered with the Fermi Gamma-ray Burst Monitor. The X-Calibur, NICER, and Swift observations reveal a pulse profile strongly dominated by one main peak, and the NICER and Swift data show strong variation of the profile from pulse to pulse. The X-Calibur observations constrain for the first time the linear polarization of the 15-35 keV emission from a highly magnetized accreting neutron star, indicating a polarization degree of % (90% confidence limit) averaged over all pulse phases. We discuss the spin up and the X-ray spectral and polarimetric results in the context of theoretical predictions. We conclude with a discussion of the scientific potential of future observations of highly magnetized neutron stars with the more sensitive follow-up mission XL-Calibur.

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.