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

TL;DR

This study reveals rapid polarization angle changes during the 2022 outburst of XTE J1701-462, indicating swift transformations in the neutron star's corona geometry, with implications for understanding accretion processes.

Contribution

First time time-resolved polarimetric analysis of XTE J1701-462 during outburst, showing rapid polarization angle variation linked to corona geometry changes.

Findings

Polarization angle varied significantly over time in the normal branch.

Polarization degree remained constant at around 2%.

Rapid PA variation caused depolarization in time-averaged data.

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.