First spectropolarimetric observation of the neutron star low-mass X-ray binary GX 3+1

TL;DR

This study presents the first simultaneous X-ray spectropolarimetric observation of the neutron star low-mass X-ray binary GX 3+1, finding low polarization consistent with a low-inclination accretion system.

Contribution

It provides the first spectropolarimetric data for GX 3+1 and constrains the system's inclination using spectral features, enhancing understanding of neutron star accretion geometries.

Findings

No significant polarization detected (upper limit 1.3%).

System inclination estimated at approximately 36 degrees.

Spectral properties align with other atoll sources observed by IXPE.

Abstract

We report the first simultaneous X-ray spectropolarimetric observation of the bright atoll neutron star low-mass X-ray binary GX 3+1, performed by the Imaging X-ray Polarimetry Explorer (IXPE) joint with NICER and NuSTAR. The source does not exhibit significant polarization in the 2-8 keV energy band, with an upper limit of 1.3% at a 99% confidence level on the polarization degree. The observed spectra can be well described by a combination of thermal disk emission, the hard Comptonization component, and reflected photons off the accretion disk. In particular, from the broad Fe K$\alpha$ line profile, we were able to determine the inclination of the system ($i \approx 36^\circ$), which is crucial for comparing the observed polarization with theoretical models. Both the spectral and polarization properties of GX 3+1 are consistent with those of other atoll sources observed by IXPE. Therefore, we may expect a similar geometrical configuration for the accreting system and the hot Comptonizing region. The low polarization is also consistent with the low inclination of the system.