A strong X-ray polarization signal from the magnetar 1RXS J170849.0-400910

TL;DR

This study reports the first X-ray polarization measurements of the magnetar 1RXS J170849.0-400910, revealing high polarization levels that increase with energy and vary with spin phase, providing insights into its magnetic and surface properties.

Contribution

It presents the first polarization data of this magnetar using IXPE, combined with Swift and NICER observations, revealing energy-dependent polarization and phase variations that inform models of its surface and magnetic structure.

Findings

Polarization degree reaches ~35% in 2-8 keV range.

Polarization increases from ~20% at 2-3 keV to ~80% at 6-8 keV.

Polarization angle remains constant across energies.

Abstract

Magnetars are the most strongly magnetized neutron stars, and one of the most promising targets for X-ray polarimetric measurements. We present here the first Imaging X-ray Polarimetry Explorer (IXPE) observation of the magnetar 1RXS J170849.0-400910, jointly analysed with a new Swift observation and archival NICER data. The total (energy and phase integrated) emission in the 2-8 keV energy range is linerarly polarized, at a ~35% level. The phase-averaged polarization signal shows a marked increase with energy, ranging from ~20% at 2-3 keV up to ~80% at 6-8 keV, while the polarization angle remain constant. This indicates that radiation is mostly polarized in a single direction. The spectrum is well reproduced by a combination of either two thermal (blackbody) components or a blackbody and a power law. Both the polarization degree and angle also show a variation with the spin phase, and the former is almost anti-correlated with the source counts in the 2-8 keV and 2-4 keV bands. We discuss the possible implications and interpretations, based on a joint analysis of the spectral, polarization and pulsation properties of the source. A scenario in which the surface temperature is not homogeneous, with a hotter cap covered by a gaseous atmosphere and a warmer region in a condensed state, provides a satisfactory description of both the phase- and energy-dependent spectro-polarimetric data. The (comparatively) small size of the two emitting regions, required to explain the observed pulsations, does not allow to reach a robust conclusion about the presence of vacuum birefringence effects.