XL-CALIBUR

XL-Calibur is a collaboration amongst scientists from the United States, Japan and Sweden, led by Principal Investigator Henric Krawczynski from Washington University in St. Louis. The mission is a balloon-borne second-generation (X-ray focusing) Compton polarimeter operational in the energy range ~15-80 keV. Astrophysical sources for observation include pulsar/wind nebula systems such as the Crab and X-ray binary systems like Cygnus X-1. X-rays are focused using the spare mirror from the Hitomi mission. Sources are acquired and tracked by the NASA-developed Wallops Arc Second Pointer (WASP). For mitigating in-flight backgrounds, a fully active anticoincidence shield is employed, developed at the KTH Royal Institute of Technology in Sweden. The instrument is flown on Long Duration Balloon (LDB) flights launched from the Esrange Space Center (Sweden) or McMurdo (Antarctica) as part of NASA’s Scientific Ballooning program.

The main objective of XL-Calibur is to measure polarization in hard X-rays for a selection of bright sources. Measurements address the following science goals:

• Constrain the shape and physical properties of the X-ray-bright accretion-disk coronas of mass-accreting stellar-mass black holes and neutron stars
• Test predictions of strong-field Quantum Electrodynamics by using polarization measurements of the X-rays from strongly magnetized neutron stars
• Identify the origin of X-rays from rotation-powered neutron stars like the Crab pulsar and accretion-powered pulsars like in GX301-2

XL-Calibur was successfully launched from the Esrange Space Center (Kiruna, Sweden) on July 9th, 2024. Following circumpolar winds in the westerly direction, the payload landed safely with a parachute near Kugluktuk (Canada) about five days later, on July 14th. The Crab system (pulsar and wind nebula) and Cygnus X-1 black (X-ray binary system) were successfully observed. A second flight is foreseen to take place from McMurdo, Antarctica. Such a flight can afford up to 8-55 days of observations of ~4-6 sources, as well as possible targets of opportunity (flaring/transient sources).

XL-Calibur observations are conducted in conjunction with other observatories, i.e., NASA’s Imaging X-ray Polarimetry Explorer (IXPE), the Neutron Star Interior Composition Explorer Mission (NICER), and the Nuc lear Spectroscopic Telescope Array (NuSTAR) mission.

XL-Calibur data are delivered and hosted by HEASARC.