IXPE and XMM-Newton observations of the Soft Gamma Repeater SGR 1806-20

TL;DR

This study combines IXPE and XMM-Newton observations of SGR 1806-20, revealing low persistent flux and a tentative polarization detection at 4-5 keV, supporting models of thermal surface emission reprocessed by magnetospheric scattering.

Contribution

First joint IXPE and XMM-Newton observations of SGR 1806-20, providing new polarization measurements and insights into its emission mechanisms.

Findings

No significant polarization detected outside 4-5 keV range.

Source at an all-time-low persistent flux level.

Polarization at 4-5 keV suggests thermal surface emission reprocessed by magnetosphere.

Abstract

Recent observations with the Imaging X-ray Polarimetry Explorer (IXPE) of two anomalous X-ray pulsars provided evidence that X-ray emission from magnetar sources is strongly polarized. Here we report on the joint IXPE and XMM-Newton observations of the soft {\gamma}-repeater SGR 1806-20. The spectral and timing properties of SGR 1806-20 derived from XMM-Newton data are in broad agreement with previous measurements; however, we found the source at an all-time-low persistent flux level. No significant polarization was measured apart from the 4-5 keV energy range, where a probable detection with PD=31.6\pm 10.5% and PA=-17.6\pm 15 deg was obtained. The resulting polarization signal, together with the upper limits we derive at lower and higher energies 2-4 and 5-8 keV, respectively) is compatible with a picture in which thermal radiation from the condensed star surface is reprocessed by resonant Compton scattering in the magnetosphere, similar to what proposed for the bright magnetar 4U 0142+61.