%D 2024 %N 2 %A F La Monaca %A A Di Marco %A J Poutanen %A M Bachetti %A SE Motta %A A Papitto %A M Pilia %A F Xie %A S Bianchi %A A Bobrikova %A E Costa %A W Deng %A MY Ge %A G Illiano %A SM Jia %A H Krawczynski %A EV Lai %A K Liu %A G Mastroserio %A F Muleri %A J Rankin %A P Soffitta %A A Veledina %A F Ambrosino %A M Del Santo %A W Chen %A JA Garcia %A P Kaaret %A TD Russell %A WH Wei %A SN Zhang %A C Zuo %A Z Arzoumanian %A M Cocchi %A A Gnarini %A R Farinelli %A K Gendreau %A F Ursini %A MC Weisskopf %A S Zane %A I Agudo %A LA Antonelli %A L Baldini %A WH Baumgartner %A R Bellazzini %A SD Bongiorno %A R Bonino %A A Brez %A N Bucciantini %A F Capitanio %A S Castellano %A E Cavazzuti %A CT Chen %A S Ciprini %A A De Rosa %A E Del Monte %A L Di Gesu %A N Di Lalla %A I Donnarumma %A V Doroshenko %A M DovÄiak %A SR Ehlert %A T Enoto %A Y Evangelista %A S Fabiani %A R Ferrazzoli %A S Gunji %A K Hayashida %A J Heyl %A W Iwakiri %A SG Jorstad %A V Karas %A F Kislat %A T Kitaguchi %A JJ Kolodziejczak %A L Latronico %A I Liodakis %A S Maldera %A A Manfreda %A F Marin %A A Marinucci %A AP Marscher %A HL Marshall %A F Massaro %A G Matt %A I Mitsuishi %A T Mizuno %A M Negro %A CY Ng %A SL O’Dell %A N Omodei %A C Oppedisano %A GG Pavlov %A AL Peirson %A M Perri %A M Pesce-Rollins %A PO Petrucci %A A Possenti %A S Puccetti %A BD Ramsey %V 960 %T Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1 %O Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence, https://creativecommons.org/licenses/by/4.0/. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. %X 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. %L discovery10186104 %J The Astrophysical Journal Letters