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

Andrea Gnarini, Ruben Farinelli, Francesco Ursini, Stefano Bianchi, Fiamma Capitanio, Giorgio Matt, Mason Ng, Antonella Tarana, Anna Bobrikova, Massimo Cocchi, Sergio Fabiani, Philip Kaaret, Juri Poutanen, Swati Ravi

Published 2024-11-15Version 1

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.