Discovery of High X-Ray Polarization from the Neutron Star Low-Mass X-Ray Binary Cyg X-2 in the Horizontal Branch

TL;DR

This study reports a high-significance X-ray polarization detection of the neutron-star LMXB Cyg X-2 in the Horizontal Branch using IXPE (2–8 keV) simultaneous with NuSTAR spectroscopy. Model-independent analysis shows $PD = $ $4.5\%\pm0.3\%$ with $PA = 128^\circ\pm2^\circ$, and an energy-dependent increase of $PD$ up to $9.9\%\pm2.8\%$ in the 7–8 keV band, with no PA rotation. Spectropolarimetric modeling of the joint IXPE+NuSTAR data with a disk+Comptonization+reflection model reveals that the polarization is dominated by Comptonized photons, but the measured high $PD$ cannot be fully explained by standard spreading-layer geometries; a combination of a highly polarized reflected component and a moderately polarized spreading layer or wind likely contributes. The results, consistent with trends seen in other Z-sources, challenge simple BL/SL polarization scenarios and underscore the need for advanced polarization models of disk reflection and equatorial winds to interpret X-ray polarization in accreting neutron stars. These findings demonstrate the power of X-ray spectro-polarimetry to constrain the geometry and emission processes in neutron-star LMXBs and motivate future high-resolution polarization studies and modeling efforts.

Abstract

We present results from simultaneous X-ray polarimetric and spectroscopic observations of the bright neutron star low-mass X-ray binary Cyg X-2, performed by the Imaging X-ray Polarimetry Explorer (IXPE) and the Nuclear Spectroscopic Telescope Array (NuSTAR). IXPE detected significant polarization (15 sigma) from the source in the 2-8 keV energy band with an average polarization degree (PD) of 4.5% +/- 0.3% and a polarization angle (PA) of 128 +/- 2 degrees as the source moved along the horizontal branch of its Z-track. The PD increases with energy reaching 9.9% +/- 2.8% in the 7-8 keV band, with no evidence for energy-dependent variation in the PA. The PA is roughly consistent with previous measurements obtained during the normal and flaring branches and also with the known radio jet axis. From spectropolarimetric analysis, the main contribution to the polarized radiation is due to Comptonized photons, but the polarization is higher than predicted in typical spreading layer geometries. The observed high polarization may be due to a combination of a highly polarized reflected component and a moderately polarized spreading layer on the neutron star surface or produced by electron scattering in an equatorial wind.

Figures (7)

• Figure 1: IXPE and NuSTAR light curves and hardness ratios of Cyg X-2. The first and third panels show the IXPE (2--8 keV) and the background subtracted NuSTAR (3--20 keV) light curves in count s$^{-1}$. The second and fourth panels show the IXPE (5--8 keV/3--5 keV) and the NuSTAR (10--20 keV/6--10 keV) hard colors. Each IXPE and NuSTAR time bin corresponds to 200 s.
• Figure 2: NuSTAR hardness-intensity diagram of Cyg X-2 from observations simultaneous with IXPE. The intensity (in count s$^{-1}$) is considered in the 3--20 keV range, while the hard color is defined as the ratio of the counts in the 10--20 keV/6--10 keV bands, after background subtraction. Gray points represent the 2022 observation, while the red ones highlight the new NuSTAR observation in 2025. Each point corresponds to 200 s.
• Figure 3: Polarization contours in the 2--4 keV, 4--6 keV and 6--8 keV band at the 68%, 90% and 99% confidence levels obtained with the ixpe_protractor task. The gray region highlights the direction of the radio jet Spencer2013, and the black, dashed lines indicate the radio jet direction and the orthogonal direction.
• Figure 4: Polarization degree (top) and angle (bottom) versus energy in 1 keV bins. The black points correspond to 2025 May 29 observation (i.e., HB) and the red points are the sum of 2022 April 30 and 2022 May 2 observations (i.e., NB and FB; Gnarini2025). Solid lines in the upper panel represent the best linear fits. Dashed lines in the lower panel correspond to the average PA in the 2--8 keV range, with the associated $1\sigma$ error highlighted by the shaded regions.
• Figure 5: IXPE (2--8 keV) and NuSTAR (3--30 keV) deconvolved spectra with the resulting best-fit model and the corresponding residuals in units of $\sigma$. The model is composed of diskbb (dash-dotted lines), thcomp*bbodyrad (dashed lines), and relxillNS (dotted lines) components. The inset shows the residuals without the reflection component, highlighting the Fe K$\alpha$ line profile.