@article{discovery10186104, note = {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.}, year = {2024}, volume = {960}, title = {Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1}, month = {January}, number = {2}, journal = {The Astrophysical Journal Letters}, issn = {2041-8205}, author = {La Monaca, F and Di Marco, A and Poutanen, J and Bachetti, M and Motta, SE and Papitto, A and Pilia, M and Xie, F and Bianchi, S and Bobrikova, A and Costa, E and Deng, W and Ge, MY and Illiano, G and Jia, SM and Krawczynski, H and Lai, EV and Liu, K and Mastroserio, G and Muleri, F and Rankin, J and Soffitta, P and Veledina, A and Ambrosino, F and Del Santo, M and Chen, W and Garcia, JA and Kaaret, P and Russell, TD and Wei, WH and Zhang, SN and Zuo, C and Arzoumanian, Z and Cocchi, M and Gnarini, A and Farinelli, R and Gendreau, K and Ursini, F and Weisskopf, MC and Zane, S and Agudo, I and Antonelli, LA and Baldini, L and Baumgartner, WH and Bellazzini, R and Bongiorno, SD and Bonino, R and Brez, A and Bucciantini, N and Capitanio, F and Castellano, S and Cavazzuti, E and Chen, CT and Ciprini, S and De Rosa, A and Del Monte, E and Di Gesu, L and Di Lalla, N and Donnarumma, I and Doroshenko, V and Dov{\vc}iak, M and Ehlert, SR and Enoto, T and Evangelista, Y and Fabiani, S and Ferrazzoli, R and Gunji, S and Hayashida, K and Heyl, J and Iwakiri, W and Jorstad, SG and Karas, V and Kislat, F and Kitaguchi, T and Kolodziejczak, JJ and Latronico, L and Liodakis, I and Maldera, S and Manfreda, A and Marin, F and Marinucci, A and Marscher, AP and Marshall, HL and Massaro, F and Matt, G and Mitsuishi, I and Mizuno, T and Negro, M and Ng, CY and O'Dell, SL and Omodei, N and Oppedisano, C and Pavlov, GG and Peirson, AL and Perri, M and Pesce-Rollins, M and Petrucci, PO and Possenti, A and Puccetti, S and Ramsey, BD}, url = {https://doi.org/10.3847/2041-8213/ad132d}, 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\% {$\pm$} 0.2\% and a polarization angle of 8o {$\pm$} 6o 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 {\ensuremath{<}}3.2\% at 90\% confidence level, compatible with expectations for an electron scattering dominated optically thick atmosphere at the Sco X-1 inclination of {$\sim$}44o; for the higher-energy Comptonized component, we obtain a polarization of 1.3\% {$\pm$} 0.4\%, in agreement with expectations for a slab of Thomson optical depth of {$\sim$}7 and an electron temperature of {$\sim$}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.} }