Polarized X-Rays Detected from the Anomalous X-Ray Pulsar 1E 2259+586

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are believed to be associated with magnetars, which have extremely strong magnetic fields. Recently, with the operation of the Imaging X-ray Polarimetry Explorer (IXPE), the polarization information of two AXPs and one SGR have been investigated. In this work, we report the observational results of the fourth magnetar, 1E 2259+586, with IXPE, and perform a joint analysis with observations from Neutron Star Interior Composition Explorer. We find that the emission from 1E 2259+586 is linearly polarized, with a polarization degree (5.3% +/- 1.3%) and a polarization angle -77 degrees +/- 7 degrees in the 2-8 keV energy range. Additionally, both the polarization degree and polarization angle exhibit variability with the pulse phase, and there is a hint of anticorrelation between the polarization degree and the flux, which is similar to AXP 1RXS J170849.0-400910. The phase-dependent polarization angle displays a sinusoidal profile and can be well fitted with the rotating vector model, indicating that the magnetic dipole field dominated the magnetic structure of the pulsar, and the variation in the polarization angle was modulated by the pulsar's rotation.