A polarimetrically oriented X-ray stare at the accreting pulsar EXO 2030+375
Authors: Malacaria Christian , Heyl Jeremy , Doroshenko Victor , Tsygankov Sergey S. , Poutanen Juri , Forsblom Sofia V. , Capitanio Fiamma , Di Marco Alessandro , Du Yujia , Ducci Lorenzo , La Monaca Fabio , Lutovinov Alexander A. , Marshall Herman L. , Mereminskiy Ilya A. , Molkov Sergey V. , Mushtukov Alexander A. , Ng Mason , Petrucci Pierre-Olivier , Santangelo Andrea , Shtykovsky Andrey E. , Suleimanov Valery F. , 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. , 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 , 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: May 2023

Accreting X-ray pulsars (XRPs) are presumed to be ideal targets for polarization measurements, as their high magnetic field strength is expected to polarize the emission up to a polarization degree of ∼80%. However, such expectations are being challenged by recent observations of XRPs with the Imaging X-ray Polarimeter Explorer (IXPE). Here, we report on the results of yet another XRP, namely, EXO 2030+375, observed with IXPE and contemporarily monitored with Insight-HXMT and SRG/ART-XC. In line with recent results obtained with IXPE for similar sources, an analysis of the EXO 2030+375 data returns a low polarization degree of 0%–3% in the phase-averaged study and a variation in the range of 2%–7% in the phase-resolved study. Using the rotating vector model, we constrained the geometry of the system and obtained a value of ∼60° for the magnetic obliquity. When considering the estimated pulsar inclination of ∼130°, this also indicates that the magnetic axis swings close to the observer’s line of sight. Our joint polarimetric, spectral, and timing analyses hint toward a complex accreting geometry, whereby magnetic multipoles with an asymmetric topology and gravitational light bending significantly affect the behavior of the observed source.