X-ray Dips and Polarization Angle Swings in GX 13+1

TL;DR

This study reports on X-ray polarization measurements of GX 13+1 during dips, revealing variable polarization degrees and angles linked to spectral changes, providing insights into the accretion environment around the neutron star.

Contribution

First polarization measurements of GX 13+1 during dips, showing how polarization properties vary with spectral states and offering new constraints on accretion models.

Findings

Polarization degree up to ~4% during dips.

70-degree swing in polarization angle across dips.

Correlation between spectral hardness and polarization properties.

Abstract

We present the result from the April 2024 observation of the low-mass X-ray binary GX 13+1 with the Imaging X-ray Polarimetry Explorer (IXPE), together with NICER and Swift-XRT coordinated observations. Two light curve dips were observed; during them, the harder Comptonized spectral component was dominant and the polarization degree higher than in the softer, off-dip intervals. Through a joint analysis of the three IXPE observations, which also included the dip from the first observation, we demonstrate that the polarization properties varied in response to the intensity and spectral hardness changes associated with the dips. The polarization degree attained values up to ~4%. The polarization angle showed a swing of ~70{\deg} across the dip and off-dip states, comparable to the continuous rotation seen during the first IXPE observation. We discuss these results in the context of models for polarized emission from the accretion disk and the boundary/spreading layer on the neutron star surface. We also draw attention to the role that an extended accretion disk corona or disk wind can play in generating high polarization degrees and, possibly, swings of the polarization angle.