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
Q. Abarr, M. Baring, B. Beheshtipour, M. Beilicke, G. de Geronimo, P. Dowkontt, M. Errando, V. Guarino, N. Iyer, F. Kislat, M. Kiss, T. Kitaguchi, H. Krawczynski, J. Lanzi, S. Li, L. Lisalda, T. Okajima, M. Pearce, L. Press, B. Rauch, D. Stuchlik, H. Takahashi, J. Tang, N. Uchida, A. West, P. Jenke, H. Krimm, A. Lien, C. Malacaria, J. M. Miller, C. Wilson-Hodge
Abstract
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 % (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 .