Decoding Cygnus X-2: The Critical Role of Reflection in IXPE Data

TL;DR

This work investigates the origin of X-ray polarization in Cygnus X-2 by combining IXPE polarimetry with quasi-simultaneous NuSTAR, NICER, and INTEGRAL spectroscopy. The authors implement full relativistic disk reflection in a spectro-polarimetric framework, showing that reflection must contribute to the polarization signal in addition to Comptonization, with the total 2–8 keV polarization around $p\approx$1.4% and a polarization angle near the jet direction. They test several reflection models (reflionx variants and xillver/relxill variants) and find that, although the inferred disk inclination can vary by model, a highly polarized reflection component (approximately $\text{PD}_{\rm refl}$ in the range $13$–$30\%$ for plausible $\text{PD}_{\rm comp}$ values) is required to explain the IXPE data. The results underscore the importance of including the full reflection spectrum in spectro-polarimetric studies and suggest that system geometry, particularly disk inclination, plays a critical role in shaping the observed polarization. These findings have broad implications for understanding accretion geometry in neutron-star X-ray binaries and for future modeling of reflected, polarized X-ray emission.

Abstract

We present a spectro-polarimetric re-analysis of the first IXPE observation of Cygnus X-2 which we determine to be mainly in the normal branch, from quasi-simultaneous observations with NuSTAR, NICER, and INTEGRAL. We measure the hard X-ray polarization angle and find it to be consistent with the previously measured position angle of the radio jet. Leveraging NuSTAR's detection of both the relativistic Fe K emission line and the Compton hump, we constrain the flux contribution of the reflected emission from the inner accretion disk to be 10% of the total X-ray flux in the IXPE energy band. Unlike previous studies that modeled only the Fe K emission line, we fit the full-band reflection spectrum using a fully relativistic disk model. There is strong degeneracy between the Comptonized and reflection components. Given that the Comptonized component is not expected to be highly polarized, a polarization degree of approximately 20% for the reflection component could explain the X-ray polarization data from IXPE. We also discuss the disk inclination angle inferred from our spectro-polarimetric modeling, as well as other possible explanations for the data.

Figures (9)

• Figure 1: (Left) Light curves of Cygnus X-2 from IXPE (DU1), NuSTAR (FPMA), NICER and INTEGRAL (JEM-X1). The consecutive observations of IXPE and NICER are distinguished using different colors. (Right) Power spectral density for the two NICER observations.
• Figure 2: The color-color diagram (left) and hardness-intensity diagram (right) plotted with the 100-s binned NuSTAR/FPMA lightcurves. All avaliable NuSTAR data are included. The observation analyzed in this work is shown in red. Other observations are color-coded based on their observation IDs.
• Figure 3: The best-fit model components and residuals for Model 1 (left) and Model 2.5 (right). Colors are coded to refer: (b) NuSTAR-FPMA (red), NuSTAR-FPMB (green), the NICER observation on April 30 (blue) and May 01 (orange); (c) INTEGRAL ISGRI (purple), JEM-X1 (cyan) and JEM-X2 (pink); (d,e,f) IXPE DU1 (yellow), DU2 (light pink) and DU3 (teal).
• Figure 4: Constraints on the PA and PD in the 2--8 keV band. Contours are shown at the 68%, 90% and 99% confidence levels. Black contours represent the result of our spectro-polarimetric analysis using XSPEC. The orange-magenta-violet contours are for the pcube algorithm.
• Figure 5: Reflection features in the NuSTAR data of Cygnus X-2. Data are fitted to a simple absorbed continuum (see the text) in the 3--4 keV and 8--10 keV and 20--40 keV bands, ignoring the bands where the reflection component is strong. Black and red colors represent data from FPMA and FPMB, respectively. The plot is for illustration purpose only.