Performance of the X-Calibur Hard X-Ray Polarimetry Mission during its 2018/19 Long-Duration Balloon Flight

X-Calibur is a balloon-borne telescope that measures the polarization of high-energy X-rays in the 15--50keV 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.Comment: 19 pages, 31 figures, submitted to Astropart. Phy

PoGOLite - A High Sensitivity Balloon-Borne Soft Gamma-ray Polarimeter

We describe a new balloon-borne instrument (PoGOLite) capable of detecting 10% polarisation from 200mCrab point-like sources between 25 and 80keV in one 6 hour flight. Polarisation measurements in the soft gamma-ray band are expected to provide a powerful probe into high-energy emission mechanisms as well as the distribution of magnetic fields, radiation fields and interstellar matter. At present, only exploratory polarisation measurements have been carried out in the soft gamma-ray band. Reduction of the large background produced by cosmic-ray particles has been the biggest challenge. PoGOLite uses Compton scattering and photo-absorption in an array of 217 well-type phoswich detector cells made of plastic and BGO scintillators surrounded by a BGO anticoincidence shield and a thick polyethylene neutron shield. The narrow FOV (1.25msr) obtained with well-type phoswich detector technology and the use of thick background shields enhance the detected S/N ratio. Event selections based on recorded phototube waveforms and Compton kinematics reduce the background to that expected for a 40-100mCrab source between 25 and 50keV. A 6 hour observation on the Crab will differentiate between the Polar Cap/Slot Gap, Outer Gap, and Caustic models with greater than 5 sigma; and also cleanly identify the Compton reflection component in the Cygnus X-1 hard state. The first flight is planned for 2010 and long-duration flights from Sweden to Northern Canada are foreseen thereafter.Comment: 11 pages, 11 figures, 2 table

The PoGOLite balloon-borne soft gamma-ray polarimeter

PoGOLite is a balloon-borne experiment that will measure the polarization of soft gamma-rays between 25 keV and 80 keV through detection of coincident Compton scattering and absorption in a close-packed array of 217 well-type phoswich detector cells. The potential observation targets include pulsars, accreting compact objects and astrophysical jets. The polarization properties of such radiation can reveal important new information on the geometry, magnetic fields and emission mechanisms of these sources.Publication written by M. Kiss (sole credited author) on behalf of the PoGOLite collaboration. QC 20120209</p

Studies of PoGOLite performance and background rejection capabilities

The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne instrument capable of measuring as low as 10% polarization from a 200 mCrab source in a sixhour °ight. A wide array of sources can be studied, including pulsars, neutron stars, accretion discs and jets from active galactic nuclei. The two new observational parameters provided by such measurements, polarization angle and degree, will allow these objects to be studied in a completely new way, providing information both about the emission mechanisms and the geometries of the emitting objects. The instrument measures anisotropies in azimuthal scattering angles of gammarays with a close-packed array of 217 well-type phoswich detector cells (PDCs) by coincident detection of Compton scattering and photoelectric absorption. Each PDC comprises a \slow" plastic scintillator tube, a \fast" plastic scintillator rod and a BGO crystal. The fast scintillator is the main detector component, whereas the slow scintillator and the BGO crystal act as an active collimator and a bottom anticoincidence shield, respectively. The three parts are viewed by a single photomultiplier tube (PMT) and pulse shape discrimination is used to identify signals from each part. The detector array is surrounded by a 54-segment side anticoincidence shield (SAS) made of BGO crystals. Each segment is 60 cm long and consists of three crystals. A total of 187 crystals have been procured and tested for light yield, energy resolution, dimensions and surface ¯nish. All crystals have been found to be of excellent quality and measured characteristics have been within speci¯ed limits. The performance of the instrument has also been evaluated in several beam tests with polarized synchrotron photons irradiating a prototype detector array. Front-end electronics have been tested and a modulation in the observed scattering angles has been observed in line with expectations. Geant4-based Monte Carlo simulations of the instrument performance have shown that a 10 cm thick polyethylene shield is required around the detector array in order to su±ciently reduce the background from atmospheric neutrons. To validate these simulations, a simple detector array with four plastic scintillators and three BGO crystals was irradiated with 14 MeV neutrons. The array was shielded with polyethylene, mimicking the PoGOLite instrument design. Measured results could be accurately recreated in Geant4 simulations, demonstrating that the treatment of neutron interaction processes in Geant4 is reliable.QC 2010111

The PoGO+ Balloon-Borne Hard X-ray Polarimetry Mission

The PoGO mission, including the PoGOLite Pathfinder and PoGO+, aims to provide polarimetric measurements of the Crab system and Cygnus X-1 in the hard X-ray band. Measurements are conducted from a stabilized balloon-borne platform, launched on a 1 million cubic meter balloon from the Esrange Space Center in Sweden to an altitude of approximately 40 km. Several flights have been conducted, resulting in two independent measurements of the Crab polarization and one of Cygnus X-1. Here, a review of the PoGO mission is presented, including a description of the payload and the flight campaigns, and a discussion of some of the scientific results obtained to date