Polarization properties of the weakly magnetized neutron star X-ray binary GS 1826-238 in the high soft state

TL;DR

This study used IXPE, INTEGRAL, and NICER observations to investigate the polarization and geometry of the weakly magnetized neutron star GS 1826-238 in a high soft state, setting an upper limit on polarization.

Contribution

First IXPE observation of a weakly magnetized neutron star in a high soft state, providing polarization constraints and insights into the source's geometry.

Findings

Upper limit of 1.3% polarization degree in 2-8 keV range

Constraints on the geometry and inclination angle of the neutron star

Simultaneous multi-instrument observations for comprehensive analysis

Abstract

The launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 2021 December 9 has opened a new window in X-ray astronomy. We report here the results of the first IXPE observation of a weakly magnetized neutron star, GS 1826-238, performed on 2022 March 29-31 when the source was in a high soft state. An upper limit (99.73% confidence level) of 1.3% for the linear polarization degree is obtained over the IXPE 2-8 keV energy range. Coordinated INTEGRAL and NICER observations were carried out simultaneously with IXPE. The spectral parameters obtained from the fits to the broad-band spectrum were used as inputs for Monte Carlo simulations considering different possible geometries of the X-ray emitting region. Comparing the IXPE upper limit with these simulations, we can put constraints on the geometry and inclination angle of GS 1826-238.