A polarimetrically oriented X-ray stare at the accreting pulsar EXO 2030+375
Kaaret, Philip; Wu, Kinwah; Enoto, Teruaki; Turolla, Roberto; Pavlov, George G.; Weisskopf, Martin C.; Tennant, Allyn F.; Swartz, Douglas A.; Slane, Patrick; Papitto, Alessandro; Hayashida, Kiyoshi; Vink, Jacco; Bianchi, Stefano; Matt, Giorgio; Kitaguchi, Takao; Doroshenko, Victor; Tsygankov, Sergey S.; Mushtukov, Alexander A.; Lutovinov, Alexander A.; Santangelo, Andrea; Suleimanov, Valery F.; Poutanen, Juri; Tombesi, Francesco; Mitsuishi, Ikuyuki; Mizuno, Tsunefumi; Tamagawa, Toru; Tawara, Yuzuru; Zane, Silvia; Dovčiak, Michal; Molkov, Sergey V.; Iwakiri, Wataru; Bachetti, Matteo; Tavecchio, Fabrizio; Thomas, Nicholas E.; Pesce-Rollins, Melissa; Marshall, Herman L.; Heyl, Jeremy; Ratheesh, Ajay; Marinucci, Andrea; De Rosa, Alessandra; Marin, Frédéric; Petrucci, Pierre-Olivier; Baumgartner, Wayne H.; Negro, Michela; Perri, Matteo; Taverna, Roberto; Malacaria, Christian; Ng, Mason; Muleri, Fabio; Di Marco, Alessandro; Krawczynski, Henric; Possenti, Andrea; Baldini, Luca; Agudo, Iván; Antonelli, Lucio A.; Bellazzini, Ronaldo; Bongiorno, Stephen D.; Bonino, Raffaella; Brez, Alessandro; Bucciantini, Niccolò; Capitanio, Fiamma; Castellano, Simone; Cavazzuti, Elisabetta; Ciprini, Stefano; Costa, Enrico; Del Monte, Ettore; Di Gesu, Laura; Di Lalla, Niccolò; Donnarumma, Immacolata; Ehlert, Steven R.; Evangelista, Yuri; Fabiani, Sergio; Ferrazzoli, Riccardo; Garcia, Javier A.; Gunji, Shuichi; Jorstad, Svetlana G.; Karas, Vladimir; Kislat, Fabian; Kolodziejczak, Jeffery J.; La Monaca, Fabio; Latronico, Luca; Liodakis, Ioannis; Maldera, Simone; Manfreda, Alberto; Marscher, Alan P.; Massaro, Francesco; O'Dell, Stephen L.; Omodei, Nicola; Oppedisano, Chiara; Peirson, Abel L.; Pilia, Maura; Puccetti, Simonetta; Ramsey, Brian D.; Rankin, John; Roberts, Oliver J.; Romani, Roger W.; Soffitta, Paolo; Spandre, Gloria; Trois, Alessio; Xie, Fei; Forsblom, Sofia V.; Chen, Chien-Ting; Ng, Chi-Yung; Sgrò, Carmelo; Ducci, Lorenzo; Mereminskiy, Ilya A.; Shtykovsky, Andrey E.; Du, Yujia
Switzerland, Canada, Germany, Finland, Italy, Russia, United States, United Kingdom, France, Spain, Czech Republic, Japan, Hong Kong SAR, Netherlands, China
Abstract
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.