Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1
Authors: La Monaca Fabio , Di Marco Alessandro , Poutanen Juri , Bachetti Matteo , Motta Sara Elisa , Papitto Alessandro , Pilia Maura , Xie Fei , Bianchi Stefano , Bobrikova Anna , Costa Enrico , Deng Wei , Ge Ming-Yu , Illiano Giulia , Jia Shu-Mei , Krawczynski Henric , Lai Eleonora Veronica , Liu Kuan , Mastroserio Guglielmo , Muleri Fabio , Rankin John , Soffitta Paolo , Veledina Alexandra , Ambrosino Filippo , Del Santo Melania , Chen Wei , Garcia Javier A. , Kaaret Philip , Russell Thomas D. , Wei Wen-Hao , Zhang Shuang-Nan , Zuo Chao , Arzoumanian Zaven , Cocchi Massimo , Gnarini Andrea , Farinelli Ruben , Gendreau Keith , Ursini Francesco , Weisskopf Martin C. , Zane Silvia , Agudo Iván , Antonelli Lucio A. , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Di Lalla Niccolò , Donnarumma Immacolata , Doroshenko Victor , Dovčiak Michal , Ehlert Steven R. , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Ferrazzoli Riccardo , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Karas Vladimir , Kislat Fabian , Kitaguchi Takao , Kolodziejczak Jeffery J. , Latronico Luca , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Marscher Alan P. , Marshall Herman L. , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Negro Michela , Ng Chi-Yung , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Pavlov George G. , Peirson Abel L. , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Possenti Andrea , Puccetti Simonetta , Ramsey Brian D. , Ratheesh Ajay , Roberts Oliver J. , Romani Roger W. , Sgrò Carmelo , Slane Patrick , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Wu Kinwah ,   IXPE Collaboration
Published: January 2024

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.