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

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: doroshv@astro.uni-tuebingen.de
² Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³ University of British Columbia, Vancouver BC V6T 1Z4, Canada
⁴ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁵ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁶ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁷ Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, 106 91 Stockholm, Sweden
⁸ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁹ Institut de Recherche en Astrophysique et Planétologie, UPS-OMP, CNRS, CNES, 9 Avenue du Colonel Roche, BP 44346 31028 Toulouse Cedex 4, France
¹⁰ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹¹ Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹² Space Research Institute (IKI) of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia
¹³ Department of Astrophysics, St. Petersburg State University, Universitetsky Pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
¹⁴ Instituto de Astrofísicade Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, RM, Italy
¹⁶ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹⁷ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
¹⁸ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹⁹ Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
²⁰ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
²¹ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
²² Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
²³ INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
²⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino, FI, Italy
²⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino, FI, Italy
²⁶ Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
²⁷ Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA
²⁸ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
²⁹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
³⁰ Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA
³¹ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
³² RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
³³ California Institute of Technology, Pasadena, CA 91125, USA
³⁴ Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
³⁵ Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
³⁶ International Center for Hadron Astrophysics, Chiba University, Chiba 263-8522, Japan
³⁷ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³⁸ Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
³⁹ Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
⁴⁰ Finnish Centre for Astronomy with ESO, University of Turku, 20014 Turku, Finland
⁴¹ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Strada Comunale Cinthia, 80126 Napoli, Italy
⁴² Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁴³ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁴⁴ Graduate School of Science, Division of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
⁴⁵ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁴⁶ University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁴⁷ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴⁸ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA
⁴⁹ Department of Physics, University of Hong Kong, Pokfulam, Hong Kong
⁵⁰ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16801, USA
⁵¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵² Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
⁵³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
⁵⁴ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁵⁶ Anton Pannekoek Institute for Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵⁷ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China

Received: 3 June 2023
Accepted: 30 June 2023

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.