Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry

Discovery of pulsations from a number of ultra-luminous X-ray (ULX) sources proved that accretion onto neutron stars can produce luminosities exceeding the Eddington limit by several orders of magnitude. The conditions necessary to achieve such high luminosities as well as the exact geometry of the accretion flow in the neutron star vicinity are, however, a matter of debate. The pulse phase-resolved polarization measurements that became possible with the launch of the Imaging X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry and its orientation relative to the orbital plane. They provide an avenue to test different theoretical models of ULX pulsars. In this paper we present the results of three IXPE observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2023 outburst. We find strong variations in the polarization characteristics with the pulsar phase. The average polarization degree increases from about 5% to 15% as the flux dropped by a factor of three in the course of the outburst. The polarization angle (PA) as a function of the pulsar phase shows two peaks in the first two observations, but changes to a characteristic sawtooth pattern in the remaining data set. This is not consistent with a simple rotating vector model. Assuming the existence of an additional constant polarized component, we were able to fit the three observations with a common rotating vector model and obtain constraints on the pulsar geometry. In particular, we find the pulsar angular momentum inclination with respect to the line of sight of i p = 15°–40°, the magnetic obliquity of θ p = 60°–80°, and the pulsar spin position angle of χ p ≈ −50°, which significantly differs from the constant component PA of about 10°. Combining these X-ray measurements with the optical PA, we find evidence for at least a 30° misalignment between the pulsar angular momentum and the binary orbital axis.

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Journal
Astronomy and Astrophysics
Published
2024-09-03
DOI
https://doi.org/10.1051/0004-6361/202450696
Citations
13
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
4.31

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article

Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry

Niccolò Di Lalla, S. V. Berdyugina, Sofia V. Forsblom, Fabian Kislat et al.
13 citations
Astronomy and Astrophysics
Astrophysical Phenomena and Observations
4.31
article

Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry

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

Abstract

Discovery of pulsations from a number of ultra-luminous X-ray (ULX) sources proved that accretion onto neutron stars can produce luminosities exceeding the Eddington limit by several orders of magnitude. The conditions necessary to achieve such high luminosities as well as the exact geometry of the accretion flow in the neutron star vicinity are, however, a matter of debate. The pulse phase-resolved polarization measurements that became possible with the launch of the Imaging X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry and its orientation relative to the orbital plane. They provide an avenue to test different theoretical models of ULX pulsars. In this paper we present the results of three IXPE observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2023 outburst. We find strong variations in the polarization characteristics with the pulsar phase. The average polarization degree increases from about 5% to 15% as the flux dropped by a factor of three in the course of the outburst. The polarization angle (PA) as a function of the pulsar phase shows two peaks in the first two observations, but changes to a characteristic sawtooth pattern in the remaining data set. This is not consistent with a simple rotating vector model. Assuming the existence of an additional constant polarized component, we were able to fit the three observations with a common rotating vector model and obtain constraints on the pulsar geometry. In particular, we find the pulsar angular momentum inclination with respect to the line of sight of i p = 15°–40°, the magnetic obliquity of θ p = 60°–80°, and the pulsar spin position angle of χ p ≈ −50°, which significantly differs from the constant component PA of about 10°. Combining these X-ray measurements with the optical PA, we find evidence for at least a 30° misalignment between the pulsar angular momentum and the binary orbital axis.

Astronomy and AstrophysicsVol. 691
University of Pisa (IT), Boston University (US), Yamagata University (JP), Hiroshima University (JP), University of Rome Tor Vergata (IT), Roma Tre University (IT), Louisiana State University (US), Marshall Space Flight Center (US), Centre National de la Recherche Scientifique (FR), Goddard Space Flight Center (US), Pennsylvania State University (US), Universities Space Research Association (US), University of Padua (IT), University of British Columbia (CA), University of Crete (GR), Guangxi University (CN), University of Turku (FI), Chiba University (JP), Istituto Nazionale di Fisica Nucleare (IT), University of New Hampshire (US), Stockholm University (SE), St Petersburg University (RU), Tohoku University (JP), Arcetri Astrophysical Observatory (IT), Astronomical Observatory of Rome (IT), Czech Academy of Sciences, Astronomical Institute (CZ), Agenzia Spaziale Italiana (IT), Space Research Institute (RU), Observatoire astronomique de Strasbourg (FR), Brera Astronomical Observatory (IT), Institut für Sonnenphysik (DE), University of Oxford (GB), Kavli Institute for Particle Astrophysics and Cosmology (US), Istituto Nazionale di Fisica Nucleare, Sezione di Torino (IT), Institute for Space Astrophysics and Planetology (IT), Pioneer (Japan) (JP), Istituto Nazionale di Fisica Nucleare, Sezione di Napoli (IT), Institut de Planétologie et d'Astrophysique de Grenoble (FR), RIKEN (JP), Istituto Nazionale di Fisica Nucleare, Roma Tor Vergata (IT), Center for Astrophysics Harvard & Smithsonian (US), Istituto Nazionale di Fisica Nucleare, Sezione di Firenze (IT), Astronomy and Space (AU), Osservatorio Astronomico di Cagliari (IT), Istituto Ricerche Solari Locarno (CH), Instituto de Astrofísica de Andalucía (ES), Istituto Nazionale di Fisica Nucleare, Sezione di Pisa (IT), University of Florence (IT), University College London (GB), University of Turin (IT), Università della Svizzera italiana (CH), Nagoya University (JP), Massachusetts Institute of Technology (US), Université de Strasbourg (FR), University of Tübingen (DE), University of Alabama in Huntsville (US), Sapienza University of Rome (IT), KTH Royal Institute of Technology (SE), University of Amsterdam (NL), University of Hong Kong (HK), Foundation for Research and Technology Hellas (GR), Université Grenoble Alpes (FR), Stanford University (US), The University of Osaka (JP)
National Aeronautics and Space Administration, Oak Ridge Associated Universities, UK Research and Innovation, Deutscher Akademischer Austauschdienst, Deutsche Forschungsgemeinschaft, Academy of Finland, Suomen Kulttuurirahasto, Agenzia Spaziale Italiana, NordForsk, Istituto Nazionale di Astrofisica, Istituto Nazionale di Fisica Nucleare, Marshall Space Flight Center, Goddard Space Flight Center, Natural Sciences and Engineering Research Council of Canada
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