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

¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, I-00146 Roma, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 101, 40129 Bologna, Italy
³ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
⁵ Trottier Space Institute, McGill University, 3550 rue University, Montréal, QC H3A 2A7, Canada
⁶ Department of Physics and Astronomy, FI-20014 University of Turku, Finland
⁷ INAF Osservatorio Astronomico di Cagliari, via della Scienza 5 I-09047 Selargius (CA), Italy
⁸ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁹ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

^⋆ Corresponding author; andrea.gnarini@uniroma3.it

Received: 17 October 2024
Accepted: 12 November 2024

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 Kα line profile, we were able to determine the inclination of the system (i ≈ 36°), 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.