X-ray pulsar GRO J1008−57 as an orthogonal rotator

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

Received: 13 February 2023
Accepted: 5 May 2023

Abstract

X-ray polarimetry is a unique way to probe the geometrical configuration of highly magnetized accreting neutron stars (X-ray pulsars). GRO J1008−57 is the first transient X-ray pulsar observed at two different flux levels by the Imaging X-ray Polarimetry Explorer (IXPE) during its outburst in November 2022. We find the polarization properties of GRO J1008−57 to be independent of its luminosity, with the polarization degree varying between nondetection and about 15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data with the rotating vector model allowed us to estimate the pulsar inclination (130°, which is in good agreement with the orbital inclination), the position angle (75°) of the pulsar spin axis, and the magnetic obliquity (∼74°). This makes GRO J1008−57 the first confidently identified nearly orthogonal rotator among X-ray pulsars. We discuss our results in the context of the neutron star atmosphere models and theories of the axis alignment of accreting pulsars.