The neutron star 4U 1735-44 is gobbling up matter from a companion star. The matter spirals into a flat disk, like water going down a bathtub drain. But this disk shines in X-rays, and astronomers have for the first time measured how "combed" those rays are: that is, how polarized.
Three telescopes (IXPE, NICER, and NuSTAR) observed the object simultaneously. The polarization turned out to be weak: only 1.4% with an uncertainty of 0.7%. This means the X-rays oscillate in almost all directions, not in a single plane. The upper limit on polarization is 3.5% at a confidence of 3 sigma (meaning the chance of error is less than 0.3%).
From the reflection model, the disk inclination was derived: about 40 degrees from the line of sight. If the disk is only slightly tilted, we see it almost face-on, and the polarization averages out. So the low polarization is not an anomaly but an expected result for such a viewing angle. Earlier, other similar sources studied by IXPE also showed weak polarization, and now it is clear that inclination plays a key role.
as written by the authors arXiv:2604.18048
X-ray polarimetry is a new tool capable of probing the geometry of accretion onto weakly magnetized neutron stars. Here we present the first X-ray spectropolarimetric results from coordinated observations of the atoll source 4U 1735-44, conducted with the Imaging X-ray Polarimetry Explorer (IXPE), NICER, and NuSTAR. Over the 2-8 keV energy range, we obtained a marginal detection of polarization with the polarization degree of $$1.4\%\pm0.7\%$$ and polarization angle of $$-29^\circ\pm14^\circ$$, corresponding to a $$3\sigma$$ upper limit on the polarization degree of 3.5\%. The best-fit model to describe the spectrum comprises a thermal component associated with the accretion disk, a Comptonized blackbody component, and a relativistic reflection component. From the reflection model, we infer a disk inclination of $$\sim 40^\circ$$. The spectroscopic and polarimetric properties of 4U 1735-44 are consistent with those observed in other atoll sources studied by IXPE, with its low polarization likely due to its low inclination.
The polarization of X-rays from an accretion disk is like glints on a wet road: if you look at a sharp angle, the glints are strong and directional, but from above they are almost invisible.
In the series "The Expanse," neutron stars are used as beacons for navigation, but their X-ray emission in reality can reveal the geometry of infalling matter, as in this work.