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X-ray pulsar GRO J1008−57 as an orthogonal rotator

2023; EDP Sciences; Volume: 675; Linguagem: Inglês

10.1051/0004-6361/202346134

1432-0746

Sergey S. Tsygankov, Victor Doroshenko, Alexander A. Mushtukov, Juri Poutanen, Alessandro Di Marco, Jeremy Heyl, Fabio La Monaca, Sofia V. Forsblom, Christian Malacaria, Herman L. Marshall, В. Ф. Сулейманов, Jiří Svoboda, Roberto Taverna, F. Ursini, I. Agudo, L. A. Antonelli, Matteo Bachetti, L. Baldini, W. H. Baumgartner, R. Bellazzini, S. Bianchi, Stephen D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, Fiamma Capitanio, Simone Castellano, E. Cavazzuti, Chien‐Ting Chen, S. Ciprini, E. Costa, Alessandra De Rosa, E. Del Monte, Laura Di Gesu, Niccolò Di Lalla, I. Donnarumma, Michal Dovčiak, Steven R. Ehlert, Teruaki Enoto, Y. Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Javier A. Garcia, Shuichi Gunji, Kiyoshi Hayashida, W. Iwakiri, Svetlana G. Jorstad, P. Kaaret, V. Karas, Fabian Kislat, Takao Kitaguchi, Jeffery J. Kolodziejczak, H. Krawczynski, L. Latronico, Ioannis Liodakis, S. Maldera, Alberto Manfreda, Frédéric Marin, Andrea Marinucci, Alan P. Marscher, F. Massaro, G. Matt, Ikuyuki Mitsuishi, Tsunefumi Mizuno, Fabio Muleri, Michela Negro, C.‐Y. Ng, Stephen L. O’Dell, N. Omodei, C. Oppedisano, Alessandro Papitto, George G. Pavlov, Abel L. Peirson, M. Perri, M. Pesce-Rollins, Pierre-Olivier Petrucci, M. Pilia, Andrea Possenti, Simonetta Puccetti, Brian D. Ramsey, John Rankin, Ajay Ratheesh, O. J. Roberts, Roger W. Romani, C. Sgrò, Patrick Slane, P. Soffitta, G. Spandre, Douglas A. Swartz, Toru Tamagawa, F. Tavecchio, Yuzuru Tawara, Allyn F. Tennant, Nicholas E. Thomas, Francesco Tombesi, A. Trois, R. Turolla, Jacco Vink, Martin C. Weisskopf, Kinwah Wu, Fei Xie, Silvia Zane,

Pulsars and Gravitational Waves Research

X-ray polarimetry is a unique way to probe the geometrical configuration of highly magnetized accreting neutron stars (X-ray pulsars). GRO J1008−57 is the first transient X-ray pulsar observed at two different flux levels by the Imaging X-ray Polarimetry Explorer (IXPE) during its outburst in November 2022. We find the polarization properties of GRO J1008−57 to be independent of its luminosity, with the polarization degree varying between nondetection and about 15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data with the rotating vector model allowed us to estimate the pulsar inclination (130°, which is in good agreement with the orbital inclination), the position angle (75°) of the pulsar spin axis, and the magnetic obliquity (∼74°). This makes GRO J1008−57 the first confidently identified nearly orthogonal rotator among X-ray pulsars. We discuss our results in the context of the neutron star atmosphere models and theories of the axis alignment of accreting pulsars.