Mechanism of the Three-Day Polarization-Angle Rotation

Determine the specific physical mechanism responsible for the approximately 40-degree polarization-angle rotation of 4U 1822-37 on a three-day timescale, particularly whether it is caused by evolution in the thickness or geometry of the accretion-disk bulge or by other absorbers along the line of sight.

Background

The paper reports a roughly 40-degree rotation of the polarization angle over approximately three days, accompanied by increased neutral absorption but no significant change in X-ray luminosity. Because this timescale is much longer than the 4U 1822-37 orbital period, the variation is interpreted as possible evolution of the absorbing structure rather than a consequence of orbital geometry. The authors discuss changes in the thickness or geometry of the outer-disk bulge, or evolution of other absorbers, as possible explanations, but the available observations do not distinguish among these mechanisms. They therefore identify detailed physical modeling as necessary to resolve the origin of the observed polarization-angle evolution.

References

Since this timescale is much longer than the orbital period, the PA variation involves the evolution of the absorber itself rather than the orbital geometry, and it is difficult to further distinguish the specific mechanism with the current observations alone. Therefore, these scenarios require more detailed modeling in the future.

— The First X-Ray Polarimetry of the Typical Accretion Disk Corona Source 4U 1822-37  (2608.24292 - Wu et al., 25 Aug 2026) in Section 4.2, subsection “PA variability caused by asymmetric structures”