Observations of a GX 301-2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM
Lien, A.; Li, S.; Takahashi, H.; Miller, J. M.; Beilicke, M.; Krimm, H.; Kislat, F.; Kitaguchi, T.; Krawczynski, H.; Errando, M.; Okajima, T.; Malacaria, C.; Kiss, M.; Pearce, M.; Wilson-Hodge, C.; Jenke, P.; Tang, J.; Iyer, N.; Beheshtipour, B.; Abarr, Q.; Baring, M.; de Geronimo, G.; Dowkontt, P.; Guarino, V.; Lanzi, J.; Lisalda, L.; Press, L.; Rauch, B.; Stuchlik, D.; Uchida, N.; West, A.
United States, Sweden, Japan
Abstract
The accretion-powered X-ray pulsar GX 301-2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late 2018 December, with contiguous observations by the Neutron star Interior Composition Explorer Mission (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 Gamma-ray Burst Monitor. 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 (27-27+38) % (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.