The detection of polarized X-ray emission from the magnetar 1E 2259+586

We report on IXPE, NICER, and XMM-Newton observations of the magnetar 1E 2259+586. We find that the source is significantly polarized at about or above 20 percent for all phases except for the secondary peak where it is more weakly polarized. The polarization degree is strongest during the primary minimum which is also the phase where an absorption feature has been identified previously. The polarization angle of the photons are consistent with a rotating vector model with a mode switch between the primary minimum and the rest of the rotation of the neutron star. We propose a scenario in which the emission at the source is weakly polarized (as in a condensed surface) and, as the radiation passes through a plasma arch, resonant cyclotron scattering off of protons produces the observed polarized radiation. This confirms the magnetar nature of the source with a surface field greater than about 10(15)G.