Discovery of Rapid Polarization Angle Variation During the 2022 Outburst of XTE J1701-462

Abstract

The geometry of the Comptonization corona in neutron star low-mass X-ray binaries is still unclear. We conducted time-resolved polarimetric analysis of the archival observations of XTE J1701--462 obtained with the \textit{Imaging X-ray Polarimeter Explorer} during its 2022 outburst, and found that the polarization angle (PA) varied significantly with time when the source was in the normal branch (NB), with $67\pm 8^{\circ }$ in the first epoch, $-34\pm 8^{\circ }$ in the second, and $-58\pm 8^{\circ }$ in the third, last epoch. Meanwhile, the polarization degree remained constant at around 2\%, above the minimum detectable polarization at the 99\% confidence level (MDP${}_{99}$). The rapid PA variation causes depolarization in the time-averaged data, resulting in a nondetection as reported in the literature. The rapid (intra-day) PA variation may suggest that there is a fast transformation of the corona geometry, likely switching from a slab geometry with enhanced disk emission and reflection, to a more vertically extended spreading layer geometry.

Acceptance for publication in the Astrophysical Journal