Design of the Telescope Truss and Gondola for the Balloon-Borne X-ray Polarimeter X-Calibur

Abstract

Received (to be inserted by publisher); Revised (to be inserted by publisher); Accepted (to be inserted by publisher); X-ray polarimetry has seen a growing interest in recent years. Improvements in detector technology and focusing X-ray optics now enable sensitive astrophysical X-ray polarization measurements. These measurements will provide new insights into the processes at work in accreting black holes, the emission of X-rays from neutron stars and magnetars, and the structure of AGN jets. X-Calibur is a balloon-borne hard X-ray scattering polarimeter. An X-ray mirror with a focal length of 8 m focuses X-rays onto the detector, which consists of a plastic scattering element surrounded by Cadmium-Zinc-Telluride detectors, which absorb and record the scattered X-rays. Since X-rays preferentially scatter perpendicular to their polarization direction, the polarization properties of an X-ray beam can be inferred from the azimuthal distribution of scattered X-rays. A close alignment of the X-ray focal spot with the center of the detector is required in order to reduce systematic uncertainties and to maintain a high photon detection efficiency. This places stringent requirements on the mechanical and thermal stability of the telescope structure. During the flight on a stratospheric balloon, X-Calibur makes use of the Wallops Arc-Second Pointer (WASP) to point the telescope at astrophysical sources. In this paper, we describe the design, construction, and test of the telescope structure, as well as its performance during a 25-hour flight from Ft. Sumner, New Mexico. The carbon fiber-aluminum composite structure met the requirements set by X-Calibur and its design can easily be adapted for other types of experiments, such as X-ray imaging or spectroscopic telescopes.

Difficulties of quantitative tests of the Kerr-hypothesis with X-ray observations of mass accreting black holes

“…The polarization of the thermal continuum emission of mass accreting stellar mass black holes will give new constraints on the orientation of the angular momentum vector of the inner accretion disk and improved measurements of the black hole spin parameter [108,162,102]. IXPE and balloon borne experiments like X-Calibur [96] will test hypotheses about the physical properties of the corona of accreting stellar mass and supermassive black holes [163,164,104,21] and the nature of the reflected emission [45,113]. We expect that X-ray polarization observations will allow us to validate (or falsify) the current accretion disk and corona models in a similar way as broadband X-ray observations of stellar mass and su-permassive black holes with NuSTAR have tested the reflection nature of the Fe Kα line emission (e.g.…”

XL-Calibur -- a second-generation balloon-borne hard X-ray polarimetry mission

“…The XL-Calibur truss will build on the flight-proven design of the X-Calibur truss [9]. The truss (Fig.…”

“…1, which will greatly extend polarimetric measurements in the 15-80 keV band. XL-Calibur follows on from the X-Calibur mission [6,7,8,9,10], which was flown on two engineering flights from Fort Sumner, New Mexico, USA (2014 & 2016) and as a Long Duration Balloon flight from McMurdo, Antarctica (December 2018-January 2019). Although the Antarctica flight was unexpectedly brief (∼2 days long), X-Calibur made detailed temporal and spectral observations of the accretion-powered pulsar GX 301−2, and constrained polarisation parameters [3].…”

Instrumentation and Future Missions in the Upcoming Era of X-ray Polarimetry

“…X-Calibur [106][107][108][109][110] is a balloon-born hard X-ray scattering polarimeter aimed to measure the polarization from BHs, NSs and AGNs. InFOCµS (International Focusing Optics Collaboration for µ-Crab Sensitivity) grazing incidence X-ray mirror focuses X-rays onto a 13 mm diameter scatterer made of a plastic scintillator rod.…”