A Strong X-Ray Polarization Signal from the Magnetar 1RXS J170849.0-400910
Authors: Zane Silvia , Taverna Roberto , González–Caniulef Denis , Muleri Fabio , Turolla Roberto , Heyl Jeremy , Uchiyama Keisuke , Ng Mason , Tamagawa Toru , Caiazzo Ilaria , Di Lalla Niccolò , Marshall Herman L. , Bachetti Matteo , La Monaca Fabio , Gau Ephraim , Di Marco Alessandro , Baldini Luca , Negro Michela , Omodei Nicola , Rankin John , Matt Giorgio , Pavlov George G. , Kitaguchi Takao , Krawczynski Henric , Kislat Fabian , Kelly Ruth , Agudo Iván , Antonelli Lucio A. , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Chen Chieng-Ting , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Donnarumma Immacolata , Doroshenko Victor , Dovčiak Michal , Ehlert Steven R. , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Ferrazzoli Riccardo , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Iwakiri Wataru , Jorstad Svetlana G. , Kaaret Philip , Karas Vladimir , Kolodziejczak Jeffery J. , Latronico Luca , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Marscher Alan P. , Massaro Francesco , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Ng C.-Y. , O’Dell Stephen L. , Oppedisano Chiara , Papitto Alessandro , Peirson Abel L. , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Poutanen Juri , Puccetti Simonetta , Ramsey Brian D. , Ratheesh Ajay , Roberts Oliver J. , Romani Roger W. , Sgró Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Swartz Douglas A. , Tavecchio Fabrizio , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei 
Published: February 2023

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 observation of the magnetar 1RXS J170849.0-400910, jointly analyzed 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 remains 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 anticorrelated with the source counts in the 2–8 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.