First X-Ray Polarization Measurement Confirms the Low Black Hole Spin in LMC X-3
Authors: Svoboda Jiří , Dovčiak Michal , Steiner James F. , Muleri Fabio , Ingram Adam , Yilmaz Anastasiya , Rodriguez Cavero Nicole , Marra Lorenzo , Poutanen Juri , Veledina Alexandra , Mojaver Mehrnoosh Rahbardar , Bianchi Stefano , García Javier A. , Kaaret Philip , Krawczynski Henric , Matt Giorgio , Podgorný Jakub , Weisskopf Martin C. , Kislat Fabian , Petrucci Pierre-Olivier , Brigitte Maimouna , Bursa Michal , Fabiani Sergio , Hu Kun , Chun Sohee , Mastroserio Guglielmo , Mikus̆incová Romana , Ratheesh Ajay , Romani Roger W. , Soffitta Paolo , Ursini Francesco , Zane Silvia , Agudo Iván , Antonelli Lucio A. , Bachetti Matteo , 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 , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Di Lalla Niccolò , Di Marco Alessandro , Donnarumma Immacolata , Doroshenko Victor , Ehlert Steven R. , Enoto Teruaki , Evangelista Yuri , Ferrazzoli Riccardo , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Karas Vladimír , Kitaguchi Takao , Kolodziejczak Jeffery J. , La Monaca Fabio , Latronico Luca , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Marscher Alan P. , Marshall Herman L. , Massaro Francesco , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Negro Michela , Ng Chi-Yung , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Perri Matteo , Pesce-Rollins Melissa , Pilia Maura , Possenti Andrea , Puccetti Simonetta , Ramsey Brian D. , Rankin John , Roberts Oliver J. , 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 , Xie Fei 
Published: December 2023

Abstract
X-ray polarization is a powerful tool to investigate the geometry of accreting material around black holes, allowing independent measurements of the black hole spin and orientation of the innermost parts of the accretion disk. We perform X-ray spectropolarimetric analysis of an X-ray binary system in the Large Magellanic Cloud, LMC X-3, that hosts a stellar-mass black hole, known to be persistently accreting since its discovery. We report the first detection of the X-ray polarization in LMC X-3 with the Imaging X-ray Polarimetry Explorer, and find the average polarization degree (PD) of 3.2% ± 0.6% and a constant polarization angle of −42° ± 6° over the 2–8 keV range. Using accompanying spectroscopic observations by NICER, NuSTAR, and the Neil Gehrels Swift observatories, we confirm previous measurements of the black hole spin via the X-ray continuum method, a ≈ 0.2. From polarization analysis only, we found consistent results with low black hole spin, with an upper limit of a < 0.7 at a 90% confidence level. A slight increase in the PD with energy, similar to other black hole X-ray binaries in the soft state, is suggested from the data but with a low statistical significance.