The Astrophysical Journal Letters (Jan 2024)

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,
  • Iván Agudo,
  • Lucio A. Antonelli,
  • Luca Baldini,
  • Wayne H. Baumgartner,
  • Ronaldo Bellazzini,
  • Stephen D. Bongiorno,
  • Raffaella Bonino,
  • Alessandro Brez,
  • Niccolò Bucciantini,
  • Fiamma Capitanio,
  • Simone Castellano,
  • Elisabetta Cavazzuti,
  • Chien-Ting Chen,
  • Stefano Ciprini,
  • Alessandra De Rosa,
  • Ettore Del Monte,
  • Laura Di Gesu,
  • Niccolò Di Lalla,
  • Immacolata Donnarumma,
  • Victor Doroshenko,
  • Michal Dovčiak,
  • 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,
  • Frédéric 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 Sgrò,
  • 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,
  • IXPE Collaboration

DOI
https://doi.org/10.3847/2041-8213/ad132d
Journal volume & issue
Vol. 960, no. 2
p. L11

Abstract

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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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