Probing the polarized emission from SMC X-1: The brightest X-ray pulsar observed by IXPE
Authors: Forsblom Sofia V. , Tsygankov Sergey S. , Poutanen Juri , Doroshenko Victor , Mushtukov Alexander A. , Ng Mason , Ravi Swati , Marshall Herman L. , Di Marco Alessandro , La Monaca Fabio , Malacaria Christian , Mastroserio Guglielmo , Loktev Vladislav , Possenti Andrea , Suleimanov Valery F. , Taverna Roberto , Agudo Iván , Antonelli Lucio A. , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , 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 , Donnarumma Immacolata , Dovčiak Michal , Ehlert Steven R. , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Ferrazzoli Riccardo , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Kaaret Philip , 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 , Matt Giorgio , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Muleri Fabio , 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 , Petrucci Pierre-Olivier , Pilia Maura , Puccetti Simonetta , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Roberts Oliver J. , Romani Roger W. , Sgrò Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Tavecchio Fabrizio , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei , Zane Silvia 
Published: September 2024

Recent observations of X-ray pulsars (XRPs) performed by the Imaging X-ray Polarimetry Explorer (IXPE) have made it possible to investigate the intricate details of these objects in a new way, thanks to the added value of X-ray polarimetry. Here we present the results of the IXPE observations of SMC X-1, a member of the small group of XRPs displaying super-orbital variability. SMC X-1 was observed by IXPE three separate times during the high state of its super-orbital period. The observed luminosity in the 2–8 keV energy band of L ∼ 2 × 1038 erg s−1 makes SMC X-1 the brightest XRP ever observed by IXPE. We detect significant polarization in all three observations, with values of the phase-averaged polarization degree (PD) and polarization angle (PA) of 3.2 ± 0.8% and 97° ±8° for Observation 1, 3.0 ± 0.9% and 90° ±8° for Observation 2, and 5.5 ± 1.1% and 80° ±6° for Observation 3, for the spectro-polarimetric analysis. The observed PD shows an increase over time with decreasing luminosity, while the PA decreases in decrements of ∼10°. The phase-resolved spectro-polarimetric analysis reveals significant detection of polarization in three out of seven phase bins, with the PD ranging between ∼2% and ∼10%, and a corresponding range in the PA from ∼70° to ∼100°. The pulse-phase resolved PD displays an apparent anti-correlation with the flux. Using the rotating vector model, we obtain constraints on the pulsar’s geometrical properties for the individual observations. The position angle of the pulsar displays an evolution over time supporting the idea that we observe changes related to different super-orbital phases. Scattering in the wind of the precessing accretion disk may be responsible for the behavior of the polarimetric properties observed during the high-state of SMC X-1’s super-orbital period.