An IXPE-led X-Ray Spectropolarimetric Campaign on the Soft State of Cygnus X-1: X-Ray Polarimetric Evidence for Strong Gravitational Lensing
Authors: Steiner James F. , Nathan Edward , Hu Kun , Krawczynski Henric , Dovčiak Michal , Veledina Alexandra , Muleri Fabio , Svoboda Jiri , Alabarta Kevin , Parra Maxime , Bhargava Yash , Matt Giorgio , Poutanen Juri , Petrucci Pierre-Olivier , Tennant Allyn F. , Baglio M. Cristina , Baldini Luca , Barnier Samuel , Bhattacharyya Sudip , Bianchi Stefano , Brigitte Maimouna , Cabezas Mauricio , Cangemi Floriane , Capitanio Fiamma , Casey Jacob , Rodriguez Cavero Nicole , Castellano Simone , Cavazzuti Elisabetta , Chun Sohee , Churazov Eugene , Costa Enrico , Di Lalla Niccolò , Di Marco Alessandro , Egron Elise , Ewing Melissa , Fabiani Sergio , García Javier A. , Green David A. , Grinberg Victoria , Hadrava Petr , Ingram Adam , Kaaret Philip , Kislat Fabian , Kitaguchi Takao , Kravtsov Vadim , Kubátová Brankica , La Monaca Fabio , Latronico Luca , Loktev Vladislav , Malacaria Christian , Marin Frédéric , Marinucci Andrea , Maryeva Olga , Mastroserio Guglielmo , Mizuno Tsunefumi , Negro Michela , Omodei Nicola , Podgorný Jakub , Rankin John , Ratheesh Ajay , Rhodes Lauren , Russell David M. , Šlechta Miroslav , Soffitta Paolo , Spooner Sean , Suleimanov Valery , Tombesi Francesco , Trushkin Sergei A. , Weisskopf Martin C. , Zane Silvia , Zdziarski Andrzej A. , Zhang Sixuan , Zhang Wenda , Zhou Menglei , Agudo Iván , Antonelli Lucio A. , Bachetti Matteo , Baumgartner Wayne H. , Bellazzini Ronaldo , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Chen Chien-Ting , Ciprini Stefano , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , 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 Vladimir , Kolodziejczak Jeffery J. , Liodakis Ioannis , Maldera Simone , Manfreda Alberto , Marscher Alan P. , Marshall Herman L. , Massaro Francesco , Mitsuishi Ikuyuki , Ng Chi-Yung , O’Dell Stephen L. , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Perri Matteo , Pesce-Rollins Melissa , Pilia Maura , Possenti Andrea , Puccetti Simonetta , Ramsey Brian D. , Roberts Oliver J. , Romani Roger W. , Sgrò Carmelo , Slane Patrick , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Thomas Nicholas E. , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Wu Kinwah , Xie Fei 
Published: July 2024

Abstract
We present the first X-ray spectropolarimetric results for Cygnus X-1 in its soft state from a campaign of five IXPE observations conducted during 2023 May–June. Companion multiwavelength data during the campaign are likewise shown. The 2–8 keV X-rays exhibit a net polarization degree PD = 1.99% ± 0.13% (68% confidence). The polarization signal is found to increase with energy across the Imaging X-ray Polarimetry Explorer’s (IXPE) 2–8 keV bandpass. The polarized X-rays exhibit an energy-independent polarization angle of PA = −25.°7 ± 1.°8 east of north (68% confidence). This is consistent with being aligned to Cyg X-1’s au-scale compact radio jet and its parsec-scale radio lobes. In comparison to earlier hard-state observations, the soft state exhibits a factor of 2 lower polarization degree but a similar trend with energy and a similar (also energy-independent) position angle. When scaling by the natural unit of the disk temperature, we find the appearance of a consistent trend line in the polarization degree between the soft and hard states. Our favored polarimetric model indicates that Cyg X-1’s spin is likely high (a
* ≳ 0.96). The substantial X-ray polarization in Cyg X-1's soft state is most readily explained as resulting from a large portion of X-rays emitted from the disk returning and reflecting off the disk surface, generating a high polarization degree and a polarization direction parallel to the black hole spin axis and radio jet. In IXPE’s bandpass, the polarization signal is dominated by the returning reflection emission. This constitutes polarimetric evidence for strong gravitational lensing of X-rays close to the black hole.