The First X-Ray Polarization Observation of the Black Hole X-Ray Binary 4U 1630–47 in the Steep Power-law State
Authors: Rodriguez Cavero Nicole , Marra Lorenzo , Krawczynski Henric , Dovčiak Michal , Bianchi Stefano , Steiner James F. , Svoboda Jiri , Capitanio Fiamma , Matt Giorgio , Negro Michela , Ingram Adam , Veledina Alexandra , Taverna Roberto , Karas Vladimir , Ursini Francesco , Podgorný Jakub , Ratheesh Ajay , Suleimanov Valery , Mikušincová Romana , Zane Silvia , Kaaret Philip , Muleri Fabio , Poutanen Juri , Malacaria Christian , Petrucci Pierre-Olivier , Gau Ephraim , Hu Kun , Chun Sohee , Agudo Iván , Antonelli Lucio A. , Bachetti Matteo , 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 , 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 , Fabiani Sergio , Ferrazzoli Riccardo , García Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Kislat Fabian , 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 , 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. , Romani Roger W. , Sgrò Carmelo , Slane Patrick , Spandre Gloria , Soffitta Paolo , Swartz Douglas A. , Tamagawa Toru , Tavecchio Fabrizio , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei 
Published: November 2023

Abstract
The Imaging X-ray Polarimetry Explorer (IXPE) observed the black hole X-ray binary 4U 1630–47 in the steep power-law (or very high) state. The observations reveal a linear polarization degree of the 2–8 keV X-rays of 6.8% ± 0.2% at a position angle of 21.°3 ± 0.°9 east of north (all errors at 1σ confidence level). Whereas the polarization degree increases with energy, the polarization angle stays constant within the accuracy of our measurements. We compare the polarization of the source in the steep power-law state with the previous IXPE measurement of the source in the high soft state. We find that, even though the source flux and spectral shape are significantly different between the high soft state and the steep power-law state, their polarization signatures are similar. Assuming that the polarization of both the thermal and power-law emission components are constant over time, we estimate the power-law component polarization to be 6.8%–7.0% and note that the polarization angle of the thermal and power-law components must be approximately aligned. We discuss the implications for the origin of the power-law component and the properties of the emitting plasma.