First Results from the Hard X-Ray Polarimetric Observations of the Accreting X-Ray Pulsar GX 301-2 with the X-Calibur Mission

On December 29, 2018, X-Calibur was launched from McMurdo in Antarctica for a stratospheric balloon flight. Though the flight was short, terminating on January 1, 2019, it was scientifically successful. The observations caught GX 301-2 close to the apastron at elevated flux levels. Here, we present the results; these include the 15-60 keV light cure, the hard X-ray energy spectrum, and the first constraints on the polarization of the hard X-ray emission. We also show results from simultaneous observations with the Neil Gehrels Swift and NICER X-ray telescopes. Finally, we include an outlook of what physical constraints could be obtained by observing X-ray pulsars with future X-ray spectropolarimetric observations.