First Detection of X-Ray Polarization from the Accreting Neutron Star 4U 1820−303
Authors: Di Marco Alessandro , La Monaca Fabio , Poutanen Juri , Russell Thomas D. , Anitra Alessio , Farinelli Ruben , Mastroserio Guglielmo , Muleri Fabio , Xie Fei , Bachetti Matteo , Burderi Luciano , Carotenuto Francesco , Del Santo Melania , Di Salvo Tiziana , Dovčiak Michal , Gnarini Andrea , Iaria Rosario , Kajava Jari J. E. , Liu Kuan , Middei Riccardo , O’Dell Stephen L. , Pilia Maura , Rankin John , Sanna Andrea , Eijnden Jakob van den , Weisskopf Martin C. , Bobrikova Anna , Capitanio Fiamma , Costa Enrico , Kaaret Philip , Marino Alessio , Soffitta Paolo , Ursini Francesco , Ambrosino Filippo , Cocchi Massimo , Fabiani Sergio , Marshall Herman L. , Matt Giorgio , Motta Sara Elisa , Papitto Alessandro , Stella Luigi , Tarana Antonella , Zane Silvia , Agudo Iván , Antonelli Lucio A. , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , 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 , Ehlert Steven R. , Enoto Teruaki , Evangelista Yuri , Ferrazzoli Riccardo , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Karas Vladimir , Kislat Fabian , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , Latronico Luca , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Marscher Alan P. , Massaro Francesco , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Negro Michela , Ng Chi-Yung , 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: August 2023

Abstract
This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820−303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array. The IXPE observations of 4U 1820−303 were coordinated with Swift X-ray Telescope, Neutron Star Interior Composition Explorer, and Nuclear Spectroscopic Telescope Array aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0% ± 0.5% and a polarization angle of −55° ± 7° in the 4–7 keV energy range, and a polarization degree of 10% ± 2% and a polarization angle of −67° ± 7° in the 7–8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of ≃90° at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 3σ upper limit of 50% at 7.25 GHz.