X-ray polarimetry of the accreting pulsar GX 301−2
Authors: Suleimanov Valery F. , Forsblom Sofia V. , Tsygankov Sergey S. , Poutanen Juri , Doroshenko Victor , Doroshenko Rosalia , Capitanio Fiamma , Di Marco Alessandro , González-Caniulef Denis , Heyl Jeremy , La Monaca Fabio , Lutovinov Alexander A. , Molkov Sergey V. , Malacaria Christian , Mushtukov Alexander A. , Shtykovsky Andrey E. , 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 , 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 , 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. , Marshall Herman L. , 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 , Possenti Andrea , 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 , Taverna Roberto , 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: August 2023

The phase- and energy-resolved polarization measurements of accreting X-ray pulsars (XRPs) allow us to test different theoretical models of their emission, and they also provide an avenue to determine the emission region geometry. We present the results of the observations of the XRP GX 301−2 performed with the Imaging X-ray Polarimetry Explorer (IXPE). A persistent XRP, GX 301−2 has one of the longest spin periods known: ∼680 s. A massive hyper-giant companion star Wray 977 supplies mass to the neutron star via powerful stellar winds. We did not detect significant polarization in the phase-averaged data when using spectro-polarimetric analysis, with the upper limit on the polarization degree (PD) of 2.3% (99% confidence level). Using the phase-resolved spectro-polarimetric analysis, we obtained a significant detection of polarization (above 99% confidence level) in two out of nine phase bins and a marginal detection in three bins, with a PD ranging between ∼3% and ∼10% and a polarization angle varying in a very wide range from ∼0° to ∼160°. Using the rotating vector model, we obtained constraints on the pulsar geometry using both phase-binned and unbinned analyses, finding excellent agreement. Finally, we discuss possible reasons for a low observed polarization in GX 301−2.