Tracking the X-Ray Polarization of the Black Hole Transient Swift J1727.8–1613 during a State Transition
Authors: Ingram Adam , Bollemeijer Niek , Veledina Alexandra , Dovčiak Michal , Poutanen Juri , Egron Elise , Russell Thomas D. , Trushkin Sergei A. , Negro Michela , Ratheesh Ajay , Capitanio Fiamma , Connors Riley , Neilsen Joseph , Kraus Alexander , Iacolina Maria Noemi , Pellizzoni Alberto , Pilia Maura , Carotenuto Francesco , Matt Giorgio , Mastroserio Guglielmo , Kaaret Philip , Bianchi Stefano , García Javier A. , Bachetti Matteo , Wu Kinwah , Costa Enrico , Ewing Melissa , Kravtsov Vadim , Krawczynski Henric , Loktev Vladislav , Marinucci Andrea , Marra Lorenzo , Mikušincová Romana , Nathan Edward , Parra Maxime , Petrucci Pierre-Olivier , Righini Simona , Soffitta Paolo , Steiner James F. , Svoboda Jiří , Tombesi Francesco , Tugliani Stefano , Ursini Francesco , Yang Yi-Jung , Zane Silvia , Zhang Wenda , Agudo Iván , Antonelli Lucio A. , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , 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 , Di Marco Alessandro , Donnarumma Immacolata , Doroshenko Victor , 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. , La Monaca Fabio , Latronico Luca , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marscher Alan P. , Marshall Herman L. , Massaro Francesco , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Muleri Fabio , Ng Chi-Yung , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Perri Matteo , Pesce-Rollins Melissa , Possenti Andrea , Puccetti Simonetta , Ramsey Brian D. , Rankin John , 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. , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Xie Fei ,   IXPE Collaboration
Published: June 2024

Abstract
We report on an observational campaign on the bright black hole (BH) X-ray binary Swift J1727.8–1613 centered around five observations by the Imaging X-ray Polarimetry Explorer. These observations track for the first time the evolution of the X-ray polarization of a BH X-ray binary across a hard to soft state transition. The 2–8 keV polarization degree decreased from ∼4% to ∼3% across the five observations, but the polarization angle remained oriented in the north–south direction throughout. Based on observations with the Australia Telescope Compact Array, we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher-spatial-resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long (≳10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8–1613 is a member of a hitherto undersampled subpopulation.