Are the large-scale magnetic fields of magnetars dominated by a dipolar structure, or is a significant magnetic-field twist required to explain the observations? A recent study by researchers from the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS) has found that the observed X-ray polarization of two magnetars can be explained by dipole-dominated magnetic fields, without requiring a significant global magnetic-field twist.

Magnetars are neutron stars with ultrastrong magnetic fields. Their magnetic fields are far stronger than those of ordinary neutron stars, making them important natural laboratories for studying strong-field physics in the universe. As a magnetar rotates, the projected direction of its magnetic axis changes on the sky, causing the observed X-ray polarization position angle to vary systematically with rotational phase. Phase-resolved polarimetric observations therefore provide important information for probing the radiation geometry and magnetic-field configuration of magnetars.

Dr. LI Biaopeng of the Pulsar Research Group at XAO, along with his supervisor Prof. GAO Zhifu, used phase-resolved polarimetric data from the Imaging X-ray Polarimetry Explorer (IXPE) to carry out a uniform Bayesian analysis of the radiation geometries of the magnetars 1E 2259+586 and 1E 1547.0−5408. In simple terms, Bayesian analysis uses observational data to update the credibility of different models and quantitatively assess which model is better supported by the data.

The study found that neither magnetar provides statistically significant evidence requiring a global magnetic-field twist, although the two sources show different levels of agreement between the models. The results were published in The Astrophysical Journal.

The researchers compared two magnetars with markedly different polarization properties. 1E 2259+586 shows relatively weak phase-averaged X-ray polarization, whereas 1E 1547.0−5408 exhibits a higher polarization degree and a clear variation in polarization position angle.

The analysis employed both the classical rotating vector model and a modified model that includes a first-order magnetic-field twist correction, and compared the two models using Bayesian evidence and information criteria. For 1E 2259+586, the twist-corrected model provides a somewhat better fit, but the statistical evidence is insufficient to establish a significant global twist. For 1E 1547.0−5408, the two models provide broadly comparable fits, and the classical dipole-dominated geometrical model is already able to describe its X-ray polarization position-angle variation well.

According to the researchers, this study provides a uniform Bayesian model-comparison framework for using X-ray polarization position angles to constrain magnetar radiation geometry and assess how closely their large-scale magnetic fields conform to a dipolar configuration.

They note that future X-ray polarimetric observations with higher sensitivity, together with multiepoch data, will help place tighter constraints on the magnetic-field geometry of magnetars.