X-ray polarization of Z-type neutron star low-mass X-ray binaries -- I. Model-independent, time-resolved X-ray polarimetry

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 colorsoft 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 X1 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 deg).

13 pages, 7 figures, 2 tables. Accepted for publication in Astronomy & Astrophysics

References in corpus (25)

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• X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch
• The X-ray Halo of GX5-1
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• Polarized radiation from the spreading layer of the weakly magnetized neutron stars
• X-ray spectro-polarimetric characterization of GX 340+0 in the horizontal branch: a highly inclined source?
• The Polarization of X-rays from Warped Black Hole Accretion Disks
• Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry
• Cross-correlations between soft and hard light curves depending on luminosity in the transient neutron star XTE J1701-462
• Fe K$\alpha$ and Fe K$\beta$ line detection in the NuSTAR spectrum of the ultra-bright Z-source Scorpius X-1
• Correlated spectro-polarimetric study along the Z track in XTE J1701-462 puts constraints on its coronal geometry
• Broadband spectro-temporal investigation of neutron star low-mass X-ray binary GX 349+2

Cited by in corpus (2)

• The detection of high X-ray polarization from an accretion disc corona source and its modelling via Monte Carlo radiation transfer simulation
• X-ray polarization of Z-type neutron star low-mass X-ray binaries -- II. Spectropolarimetric analysis