  The accretion-powered X-ray pulsar GX 301-2 was observed with the
balloon-borne X-Calibur hard X-ray polarimeter during late December 2018, with
contiguous observations by the 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 GBM. 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 (27+38-27)% (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.

High Energy Astrophysical Phenomena;High Energy Physics - Experiment