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 XMMNewton observations of the soft.-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 alltime 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.degrees 0)(+. 15.degrees 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.