IXPE detection of highly polarized X-rays from the magnetar 1E 1841-045

TL;DR

This paper reports the first detection of highly polarized X-ray emission from the magnetar 1E 1841-045 by IXPE, revealing energy-dependent polarization properties and providing insights into the magnetar's emission mechanisms.

Contribution

It presents the first polarization measurements of 1E 1841-045, demonstrating energy-dependent polarization degrees and spectral modeling with combined IXPE and NuSTAR data.

Findings

Polarization degree increases from ~15% at 2-3 keV to ~55% at 5.5-8 keV.

Polarization angle remains aligned with celestial North across energies.

Hard power-law component shows polarization exceeding 65%, indicating synchrotron/curvature emission.

Abstract

The Imaging X-ray Polarimetry Explorer (IXPE) observed for the first time highly polarized X-ray emission from the magnetar 1E 1841-045, targeted after a burst-active phase in August 2024. To date, IXPE has observed four other magnetars during quiescent periods, highlighting substantially different polarization properties. 1E 1841-045 exhibits a high, energy-dependent polarization degree, which increases monotonically from ~15% at 2-3 keV up to ~55% at 5.5-8 keV, while the polarization angle, aligned with the celestial North, remains fairly constant. The broadband spectrum (2-79 keV) obtained by combining simultaneous IXPE and NuSTAR data is well modeled by a blackbody and two power-law components. The unabsorbed 2-8 keV flux (~2E-11 erg/cm2/s) is about 10% higher than that obtained from archival XMM-Newton and NuSTAR observations. The polarization of the soft, thermal component does not exceed ~25%, and may be produced by a condensed surface or a bombarded atmosphere. The intermediate power law is polarized at around 30%, consistent with predictions for resonant Compton scattering in the star magnetosphere; while, the hard power law exhibits a polarization degree exceeding 65%, pointing to a synchrotron/curvature origin.