X-ray polarization of Z-type neutron star low-mass X-ray binaries

I. Model-independent, time-resolved X-ray polarimetry

¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy
² INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy
³ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius, (CA), Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 101, I-40129 Bologna, Italy

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

Received: 17 March 2025
Accepted: 26 May 2025

Abstract

Z-sources are a particular class of neutron star low-mass X-ray binaries characterized by a wide Z-like track in their hard color-soft color (or hardness-intensity) diagrams, with three branches: the horizontal (HB), the normal (NB), and the flaring branch (FB). Spectropolarimetric observations with the Imaging X-ray Polarimetry Explorer (IXPE) show that the polarization in these sources varies along the Z-track, reaching unexpectedly high values in the HB. In this work, we collected all the polarimetric results obtained so far from observations of Z-sources with IXPE, using a model-independent analysis with IXPEOBSSIM. We first performed a detailed characterization of the spectral state of each source along the Z-track using IXPE, along with the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron Star Interior Composition Explorer (NICER) data and then estimated the polarization for each branch. Although we confirm that the average polarization in the 2–8 keV band decreases moving from the HB to the NB for all three Z-sources observed in these branches, we also observe a qualitatively increasing trend from the NB to the FB. Whereas this increase is clearly significant for Cyg X-2 and Sco X-1, the polarization remains consistent at the 90% confidence level for GX 5–1 and GX 349+2, while for XTE J1701–462 and GX 340+0 only upper limits are found in the FB. For most sources, the average polarization angle in the 2–8 keV range remains consistent along the CCD; however, we observe a significant rotation for both Sco X-1 and GX 349+2 (at the 90% confidence level) as they move from the NB to the FB. In addition, we observe a significant increase in the polarization degree with energy in most of the observed Z-sources, with some also exhibiting a rotation of the polarization angle with energy (approximately by 20° −30°).