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Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1

Fabio La Monaca,Alessandro Di Marco,Juri Poutanen,Matteo Bachetti,Sara Elisa Motta,Alessandro Papitto, Maura Pilia,Fei Xie,Stefano Bianchi,Anna Bobrikova,Enrico Costa,Wei Deng,Ming-Yu Ge,Giulia Illiano,Shu-Mei Jia,Henric Krawczynski,Eleonora Veronica Lai,Kuan Liu,Guglielmo Mastroserio,Fabio Muleri,John Rankin,Paolo Soffitta,Alexandra Veledina,Filippo Ambrosino,Melania Del Santo, Wei Chen,Javier A. Garcia,Philip Kaaret,Thomas D. Russell, Wen-Hao Wei,Shuang-Nan Zhang, Chao Zuo,Zaven Arzoumanian,Massimo Cocchi,Andrea Gnarini,Ruben Farinelli,Keith Gendreau,Francesco Ursini,Martin C. Weisskopf,Silvia Zane,Ivan Agudo,Lucio A. Antonelli,Luca Baldini,Wayne H. Baumgartner,Ronaldo Bellazzini,Stephen D. Bongiorno,Raffaella Bonino,Alessandro Brez,Niccolo Bucciantini,Fiamma Capitanio,Simone Castellano,Elisabetta Cavazzuti,Chien-Ting Chen,Stefano Ciprini, Alessandra De Rosa,Ettore Del Monte,Laura Di Gesu,Niccolo Di Lalla,Immacolata Donnarumma,Victor Doroshenko,Michal Dovciak,Steven R. Ehlert,Teruaki Enoto,Yuri Evangelista,Sergio Fabiani,Riccardo Ferrazzoli,Shuichi Gunji,Kiyoshi Hayashida,Jeremy Heyl,Wataru Iwakiri,Svetlana G. Jorstad,Vladimir Karas,Fabian Kislat,Takao Kitaguchi,Jeffery J. Kolodziejczak,Luca Latronico,Ioannis Liodakis,Simone Maldera,Alberto Manfreda,Frederic Marin,Andrea Marinucci,Alan P. Marscher,Herman L. Marshall,Francesco Massaro,Giorgio Matt,Ikuyuki Mitsuishi,Tsunefumi Mizuno,Michela Negro,Chi-Yung Ng,Stephen L. O'Dell,Nicola Omodei,Chiara Oppedisano,George G. Pavlov,Abel L. Peirson,Matteo Perri,Melissa Pesce-Rollins,Pierre-Olivier Petrucci,Andrea Possenti,Simonetta Puccetti,Brian D. Ramsey,Ajay Ratheesh,Oliver J. Roberts,Roger W. Romani,Carmelo Sgro,Patrick Slane,Gloria Spandre,Douglas A. Swartz,Toru Tamagawa,Fabrizio Tavecchio,Roberto Taverna,Yuzuru Tawara,Allyn F. Tennant,Nicholas E. Thomas,Francesco Tombesi,Alessio Trois,Sergey S. Tsygankov,Roberto Turolla,Jacco Vink,Kinwah Wu

ASTROPHYSICAL JOURNAL LETTERS(2024)

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Abstract
The Imaging X-ray Polarimetry Explorer measured with high significance the X-ray polarization of the brightest Z-source, Sco X-1, resulting in the nominal 2–8 keV energy band in a polarization degree of 1.0% ± 0.2% and a polarization angle of 8° ± 6° at a 90% confidence level. This observation was strictly simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT, which allowed for a precise characterization of its broadband spectrum from soft to hard X-rays. The source has been observed mainly in its soft state, with short periods of flaring. We also observed low-frequency quasiperiodic oscillations. From a spectropolarimetric analysis, we associate a polarization to the accretion disk at <3.2% at 90% confidence level, compatible with expectations for an electron scattering dominated optically thick atmosphere at the Sco X-1 inclination of ∼44°; for the higher-energy Comptonized component, we obtain a polarization of 1.3% ± 0.4%, in agreement with expectations for a slab of Thomson optical depth of ∼7 and an electron temperature of ∼3 keV. A polarization rotation with respect to previous observations by OSO-8 and PolarLight, and also with respect to the radio-jet position angle, is observed. This result may indicate a variation of the polarization with the source state that can be related to relativistic precession or a change in the corona geometry with the accretion flow.
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Key words
Polarimetry,Spectropolarimetry,Neutron stars,Accretion,Stellar accretion disks,Low-mass x-ray binary stars
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