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arxiv: 2306.02116 · v2 · pith:WXXAOFJ3new · submitted 2023-06-03 · 🌌 astro-ph.HE

Complex variations of X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431

Victor Doroshenko , Juri Poutanen , Jeremy Heyl , Sergey S. Tsygankov , Ilaria Caiazzo , Roberto Turolla , Alexandra Veledina , Martin C. Weisskopf
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Sofia V. Forsblom Denis Gonz\'alez-Caniulef Vladislav Loktev Christian Malacaria Alexander A. Mushtukov Valery F. Suleimanov Alexander A. Lutovinov Ilya A. Mereminskiy Sergey V. Molkov Alexander Salganik Andrea Santangelo Andrei V. Berdyugin Vadim Kravtsov Anagha P. Nitindala Iv\'an Agudo Lucio A. Antonelli Matteo Bachetti Luca Baldini Wayne H. Baumgartner Ronaldo Bellazzini Stefano Bianchi Stephen D. Bongiorno Raffaella Bonino Alessandro Brez Niccol\`o Bucciantini Fiamma Capitanio Simone Castellano Elisabetta Cavazzuti Chien-Ting Chen Stefano Ciprini Enrico Costa Alessandra De Rosa Ettore Del Monte Laura Di Gesu Niccol\`o Di Lalla Alessandro Di Marco Immacolata Donnarumma Michal Dov\v{c}iak Steven R. Ehlert Teruaki Enoto Yuri Evangelista Sergio Fabiani Riccardo Ferrazzoli Javier A. Garc\'ia Shuichi Gunji Kiyoshi Hayashida Wataru Iwakiri Svetlana G. Jorstad Philip Kaaret Vladimir Karas Fabian Kislat Takao Kitaguchi Jeffery J. Kolodziejczak Henric Krawczynski Fabio La Monaca Luca Latronico Ioannis Liodakis Simone Maldera Alberto Manfreda Fr\'ed\'eric Marin Andrea Marinucci Alan P. Marscher Herman L. Marshall Francesco Massaro Giorgio Matt Ikuyuki Mitsuishi Tsunefumi Mizuno Fabio Muleri Michela Negro Chi-Yung Ng Stephen L. O'Dell Nicola Omodei Chiara Oppedisano Alessandro Papitto George G. Pavlov Abel L. Peirson Matteo Perri Melissa Pesce-Rollins Pierre-Olivier Petrucci Maura Pilia Andrea Possenti Simonetta Puccetti Brian D. Ramsey John Rankin Ajay Ratheesh Oliver J. Roberts Roger W. Romani Carmelo Sgr\`o Patrick Slane Paolo Soffitta Gloria Spandre Douglas A. Swartz Toru Tamagawa Fabrizio Tavecchio Roberto Taverna Yuzuru Tawara Allyn F. Tennant Nicholas E. Thomas Francesco Tombesi Alessio Trois Jacco Vink Kinwah Wu Fei Xie Silvia Zane
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classification 🌌 astro-ph.HE
keywords observedpulsarx-rayobservationsphasepolarizationpolarizedangle
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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.

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