X Persei is a persistent low-luminosity X-ray pulsar of period of ≈ 835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above 10 G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3–8 keV band varied from several to ∼20 per cent over the pulse with a phase dependence resembling the pulse profile. The polarization angle shows significant variation and makes two complete revolutions during the pulse period, resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky, as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination, reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008−57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered, with its value increasing from 0 at ∼2 keV to 30per cent at 8 keV. © 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.

Full list of the authors: Mushtukov, A. A.; Tsygankov, S. S.; Poutanen, J.; Doroshenko, V.; Salganik, A.; Costa, E.; Marco, A. Di; Heyl, J.; Monaca, F. La; Lutovinov, A. A.; Mereminsky, I. A.; Papitto, A.; Semena, A. N.; Shtykovsky, A. E.; Suleimanov, V. F.; Forsblom, S. V.; González-Caniulef, D.; Malacaria, C.; Sunyaev, R. A.; Agudo, I.; Antonelli, L. A.; Bachetti, M.; Baldini, L.; Baumgartner, W. H.; Bellazzini, R.; Bianchi, S.; Bongiorno, S. D.; Bonino, R.; Brez, A.; Bucciantini, N.; Capitanio, F.; Castellano, S.; Cavazzuti, E.; Chen, C. -T.; Ciprini, S.; De Rosa, A.; Del Monte, E.; Gesu, L. Di; Lalla, N. Di; Donnarumma, I.; Dovčiak, M.; Ehlert, S. R.; Enoto, T.; Evangelista, Y.; Fabiani, S.; Ferrazzoli, R.; Garcia, J. A.; Gunji, S.; Hayashida, K.; Iwakiri, W.; Jorstad, S. G.; Kaaret, P.; Karas, V.; Kislat, F.; Kitaguchi, T.; Kolodziejczak, J. J.; Krawczynski, H.; Latronico, L.; Liodakis, I.; Maldera, S.; Manfreda, A.; Marin, F.; Marscher, A. P.; Marshall, H. L.; Massaro, F.; Matt, G.; Mitsuishi, I.; Mizuno, T.; Muleri, F.; Negro, M.; Ng, C. -Y.; O'Dell, S. L.; Omodei, N.; Oppedisano, C.; Pavlov, G. G.; Peirson, A. L.; Perri, M.; Pesce-Rollins, M.; Petrucci, P. -O.; Pilia, M.; Possenti, A.; Puccetti, S.; Ramsey, B. D.; Rankin, J.; Ratheesh, A.; Roberts, O. J.; Romani, R. W.; Sgrò, C.; Slane, P.; Soffitta, P.; Spandre, G.; Swartz, D. A.; Tamagawa, T.; Tavecchio, F.; Taverna, R.; Tawara, Y.; Tennant, A. F.; Thomas, N. E.; Tombesi, F.; Trois, A.; Turolla, R.; Vink, J.; Weisskopf, M. C.; Wu, K.; Xie, F.; Zane, S.

The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency, Agenzia Spaziale Italiana (ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC) with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center ( heasarc ), at NASA Goddard Space Flight Center (GSFC). We acknowledge support from UKRI Stephen Hawking fellowship and the Netherlands Organization for Scientific Research Veni fellowship (AAM), the Academy of Finland grant nos. 333112, 349144, 349373, and 349906 (SST, JP), the Väisälä Foundation (SST), the German Academic Exchange Service (DAAD) travel grant no. 57525212 (VD, VFS), the German Research Foundation (DFG) grant no. WE 1312/53-1 (VFS), the Russian Science Foundation grant no. 19-12-00423 (AS, AAL, IAM, ANS, and AES), and the CNES fellowship grant no. (DG-C).