@article{15193,
  abstract     = {We report on Imaging X-ray polarimetry explorer (IXPE) observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminosity levels during the giant outburst in January–February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in supercritical and subcritical states with significantly different emission-region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase-resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on the pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations in the spin axis inclination, the position angle, and the magnetic colatitude by tens of degrees within the space of just a few days. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of an unpulsed polarized component in addition to the polarized radiation associated with the pulsar itself. We then show that the observed PA phase dependence in both observations can be explained with a single set of RVM parameters defining the pulsar’s geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation in the equatorial disk wind.},
  author       = {Doroshenko, Victor and Poutanen, Juri and Heyl, Jeremy and Tsygankov, Sergey S. and Caiazzo, Ilaria and Turolla, Roberto and Veledina, Alexandra and Weisskopf, Martin C. and Forsblom, Sofia V. and González-Caniulef, Denis and Loktev, Vladislav and Malacaria, Christian and Mushtukov, Alexander A. and Suleimanov, Valery F. and Lutovinov, Alexander A. and Mereminskiy, Ilya A. and Molkov, Sergey V. and Salganik, Alexander and Santangelo, Andrea and Berdyugin, Andrei V. and Kravtsov, Vadim and Nitindala, Anagha P. and Agudo, Iván and Antonelli, Lucio A. and Bachetti, Matteo and Baldini, Luca and Baumgartner, Wayne H. and Bellazzini, Ronaldo and Bianchi, Stefano and Bongiorno, Stephen D. and Bonino, Raffaella and Brez, Alessandro and Bucciantini, Niccolò and Capitanio, Fiamma and Castellano, Simone and Cavazzuti, Elisabetta and Chen, Chien-Ting and Ciprini, Stefano and Costa, Enrico and De Rosa, Alessandra and Del Monte, Ettore and Di Gesu, Laura and Di Lalla, Niccolò and Di Marco, Alessandro and Donnarumma, Immacolata and Dovčiak, Michal and Ehlert, Steven R. and Enoto, Teruaki and Evangelista, Yuri and Fabiani, Sergio and Ferrazzoli, Riccardo and García, Javier A. and Gunji, Shuichi and Hayashida, Kiyoshi and Iwakiri, Wataru and Jorstad, Svetlana G. and Kaaret, Philip and Karas, Vladimir and Kislat, Fabian and Kitaguchi, Takao and Kolodziejczak, Jeffery J. and Krawczynski, Henric and La Monaca, Fabio and Latronico, Luca and Liodakis, Ioannis and Maldera, Simone and Manfreda, Alberto and Marin, Frédéric and Marinucci, Andrea and Marscher, Alan P. and Marshall, Herman L. and Massaro, Francesco and Matt, Giorgio and Mitsuishi, Ikuyuki and Mizuno, Tsunefumi and Muleri, Fabio and Negro, Michela and Ng, Chi-Yung and O’Dell, Stephen L. and Omodei, Nicola and Oppedisano, Chiara and Papitto, Alessandro and Pavlov, George G. and Peirson, Abel L. and Perri, Matteo and Pesce-Rollins, Melissa and Petrucci, Pierre-Olivier and Pilia, Maura and Possenti, Andrea and Puccetti, Simonetta and Ramsey, Brian D. and Rankin, John and Ratheesh, Ajay and Roberts, Oliver J. and Romani, Roger W. and Sgrò, Carmelo and Slane, Patrick and Soffitta, Paolo and Spandre, Gloria and Swartz, Douglas A. and Tamagawa, Toru and Tavecchio, Fabrizio and Taverna, Roberto and Tawara, Yuzuru and Tennant, Allyn F. and Thomas, Nicholas E. and Tombesi, Francesco and Trois, Alessio and Vink, Jacco and Wu, Kinwah and Xie, Fei and Zane, Silvia},
  issn         = {1432-0746},
  journal      = {Astronomy & Astrophysics},
  keywords     = {Space and Planetary Science, Astronomy and Astrophysics},
  publisher    = {EDP Sciences},
  title        = {{Complex variations in X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431}},
  doi          = {10.1051/0004-6361/202347088},
  volume       = {677},
  year         = {2023},
}