In the binary system Cyg X-2, a neutron star pulls matter from a normal companion star. The infalling gas spirals in, heats up to millions of degrees, and shines brightly in X-rays. Astronomers pointed two space telescopes, IXPE and NuSTAR, at this object and, for the first time, measured in detail how polarized that light is.
Polarization is a property of light where its waves oscillate not randomly but predominantly in one plane. Ordinary sunlight is unpolarized, but after reflecting off water or glass, it becomes partially polarized: that's why polarized sunglasses reduce glare. In the X-ray range, polarization reveals the shape and location of the hot region around the neutron star.
For Cyg X-2, the average degree of polarization was 4.5%—a very confident detection (15 sigma). But the most interesting part: as energy increases from 2 to 8 keV, the polarization strengthens and reaches nearly 10%. Such an increase does not fit the simple "spreading layer" model on the star's surface. It seems that reflection of X-rays from the accretion disk or scattering in an equatorial wind from the disk comes into play.
This result is an important step toward understanding how matter behaves in the most extreme conditions, just a few kilometers from the neutron star's surface, where gravity and magnetic fields are unimaginably strong.
as written by the authors arXiv:2601.11718
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.
The polarization of X-rays from neutron stars is so small that before IXPE's launch in 2021, it was nearly impossible to measure — instruments lacked the sensitivity.
In Bruce Sterling's novel Schismatrix, neutron stars and their radiation play a key role in interstellar politics — though polarization is not mentioned there.