IXPE and XMM-Newton observations of the soft gamma repeater SGR 1806-20

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 γ-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% ± 10.5% and PA=−17.∘6+15.∘5−15.∘0 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 was proposed for the bright magnetar 4U 0142+61.