New polarimetric study of the galactic X-ray burster GX 13+1

TL;DR

This study presents new polarimetric observations of the galactic X-ray burster GX 13+1, revealing variable polarization and system misalignment, and emphasizes the role of the wind in the source's behavior.

Contribution

It provides the first detailed polarimetric analysis of GX 13+1 with IXPE, highlighting system misalignment and the importance of wind effects in the source's phenomenology.

Findings

Detected variable polarization at 2-5% level.

Confirmed system misalignment through polarization angle rotation.

Highlighted the role of wind in the source's behavior.

Abstract

Weakly magnetized neutron stars (WMNS) are complicated sources with challenging phenomenology. For decades, they have been studied via spectrometry and timing. It has been established that the spectrum of WMNSs consists of several components traditionally associated with the accretion disk, the boundary or spreading layer, and the wind and their interactions with each other. Since 2022, WMNSs have been actively observed with the Imaging X-ray Polarimetry Explorer (IXPE). Polarimetric studies provided new information about the behavior and geometry of these sources. One of the most enigmatic sources of the class, galactic X-ray burster GX 13+1 was first observed with IXPE in October 2023. A strongly variable polarization at the level 2-5$\%$ was detected with the source showing a rotation of the polarization angle (PA) that hinted towards the misalignment within the system. The second observation was performed in February 2024 with a complementary observation by Swift/XRT. IXPE measured an overall polarization degree (PD) of 2.5$\%$ and the PA of 24 degrees, and the Swift/XRT data helped us evaluate the galactic absorption and fit the continuum. Here we study the similarities and differences between the polarimetric properties of the source during the two observations. We confirm the expectation of the misalignment in the system and the assignment of the harder component to the boundary layer. We emphasize the importance of the wind in the system. We note the difference in the variation of polarimetric properties with energy and with time.