A Strong X-Ray Polarization Signal from the Magnetar 1RXS J170849.0-400910

Full list of authors: Zane, Silvia; Taverna, Roberto; Gonzalez-Caniulef, Denis; Muleri, Fabio; Turolla, Roberto; Heyl, Jeremy; Uchiyama, Keisuke; Ng, Mason; Tamagawa, Toru; Caiazzo, Ilaria; Di Lalla, Niccolo; 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, Ivan; Antonelli, Lucio A.; Baumgartner, Wayne H.; Bellazzini, Ronaldo; Bianchi, Stefano; Bongiorno, Stephen D.; Bonino, Raffaella; Brez, Alessandro; Bucciantini, Niccolo; 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; Dovciak, 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, Frederic; 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.; Sgro, 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.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
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. © 2023. The Author(s). Published by the American Astronomical Society.
The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana; ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC) with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF, and INFN). We thank K. C. Gendreau and Z. Arzoumanian for their help in scheduling NICER observations of the source. We acknowledge the use of public data from the Swift data archive, and we thank the Swift team for promptly scheduling a target of opportunity (ToO) observation. R.T. and R.T. acknowledge financial support from the Italian Ministry of University and Research (MUR) through grant PRIN 2017LJ39LM. D.G.-C., J.H., and I.C. acknowledge support from the Natural Sciences and Engineer Council of Canada and the Canadian Space Agency. M.N. acknowledges the support by NASA under award number 80GSFC21M0002. T.T. was supported by grant JSPS KAKENHI JP19H05609. H.K. and E.G. acknowledge NASA support under grants 80NSSC18K0264, 80NSSC22K1291, 80NSSC21K1817, and NNX16AC42G.
With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).