Tilted rotation axes in weakly magnetized neutron stars change X-ray polarization signals

Researchers have developed a new model to explain how the orientation of a neutron star affects its X-ray light. By accounting for a misalignment between the star's rotation and the binary system's axis, the study provides a framework to interpret data from the Imaging X-ray Polarimetry Explorer, revealing how the geometry of these high-energy systems imprints specific patterns on the light they emit.

Star system: Circinus X-1 A | Distance from the Sun: 31000 light years

In many binary star systems, a compact neutron star pulls matter from a companion star. This material forms a layer on the surface of the neutron star, often referred to as a spreading layer, which glows brightly in X-rays. Standard models for these systems usually assume that the rotation of the neutron star is perfectly aligned with the orbital plane of the binary system. However, new observational data suggest that this alignment is not always perfect, with some neutron stars potentially wobbling or sitting at an incline relative to their orbit.

To understand how this tilt affects observed light, the new model calculates the polarimetric properties of X-ray emissions. Polarimetry measures the orientation of light waves, a property that is sensitive to the physical geometry of the source. By analyzing how the spreading layer emits X-rays from an inclined position, the researchers established how these geometric factors alter the polarization signal compared to perfectly aligned systems. This approach allows for a more accurate interpretation of data captured by the Imaging X-ray Polarimetry Explorer (IXPE), a space telescope designed to measure the polarization of X-rays from cosmic sources.

This analytical model provides a necessary tool for future X-ray studies. It enables the comparison of theoretical predictions with real-world observations to confirm if a specific neutron star is indeed tilted. By refining how these geometrical parameters impact the detected light, researchers can better map the environments surrounding these dense, collapsed stars.

Key findings
Standard models assuming aligned rotation axes may be insufficient for explaining current X-ray polarimetric data.
A newly developed model calculates the polarimetric signature of a spreading layer on an inclined, weakly magnetized neutron star.
System orientation significantly alters the polarization of emitted X-rays compared to aligned systems.
The model provides a framework to interpret observations from the Imaging X-ray Polarimetry Explorer.

Scientific study

Breaking the symmetry: X-ray polarization from a spreading layer on a tilted weakly magnetized neutron star (external link)