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

Silvia Zane , Roberto Taverna , Denis González-Caniulef , Fabio Muleri , Roberto Turolla , Jeremy Heyl , Keisuke Uchiyama , Mason Ng , Toru Tamagawa , Ilaria Caiazzo , Niccolò Di Lalla , Herman L. Marshall , Matteo Bachetti , Fabio La Monaca , Ephraim Gau , Alessandro Di Marco , Luca Baldini , Michela Negro , Nicola Omodei , John Rankin , Giorgio Matt , George G. Pavlov , Takao Kitaguchi , Henric Krawczynski , Fabian Kislat , Ruth Kelly , Iván Agudo , Lucio A. Antonelli , Wayne H. Baumgartner , Ronaldo Bellazzini , Stefano Bianchi , Stephen D. Bongiorno , Raffaella Bonino , Alessandro Brez , Niccolò Bucciantini , Fiamma Capitanio , Simone Castellano , Elisabetta Cavazzuti , Chieng-Ting Chen , Stefano Ciprini , Enrico Costa , Alessandra de Rosa , Ettore del Monte , Laura Di Gesu , Immacolata Donnarumma , Victor Doroshenko , Michal Dovčiak , Steven R. Ehlert , Teruaki Enoto , Yuri Evangelista , Sergio Fabiani , Riccardo Ferrazzoli , Javier A. Garcia , Shuichi Gunji , Kiyoshi Hayashida , Wataru Iwakiri , Svetlana G. Jorstad , Philip Kaaret , Vladimir Karas , Jeffery J. Kolodziejczak , Luca Latronico , Ioannis Liodakis , Simone Maldera , Alberto Manfreda , Frédéric Marin (1) , Andrea Marinucci , Alan P. Marscher , Francesco Massaro , Ikuyuki Mitsuishi , Tsunefumi Mizuno , C. -Y. Ng , Stephen L. O'Dell , Chiara Oppedisano , Alessandro Papitto , Abel L. Peirson , Matteo Perri , Melissa Pesce-Rollins , Pierre-Olivier Petrucci (2) , Maura Pilia , Andrea Possenti , Juri Poutanen , Simonetta Puccetti , Brian D. Ramsey , Ajay Ratheesh , Oliver J. Roberts , Roger W. Romani , Carmelo Sgró , Patrick Slane , Paolo Soffitta , Gloria Spandre , Douglas A. Swartz , Fabrizio Tavecchio , Yuzuru Tawara , Allyn F. Tennant , Nicholas E. Thomas , Francesco Tombesi , Alessio Trois , Sergey S. Tsygankov , Jacco Vink , Martin C. Weisskopf , Kinwah Wu , Fei Xie

1 ObAS - Observatoire astronomique de Strasbourg
2 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

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.

Keywords: Polarimetry; Magnetars; 1278; 992; Astrophysics - High Energy Astrophysical Phenomena

Domains: Sciences of the Universe [physics]

Silvia Zane, Roberto Taverna, Denis González-Caniulef, Fabio Muleri, Roberto Turolla, et al.. A Strong X-Ray Polarization Signal from the Magnetar 1RXS J170849.0-400910. The Astrophysical journal letters, 2023, 944, ⟨10.3847/2041-8213/acb703⟩. ⟨insu-04031688⟩