X-ray polarimetry of the accreting pulsar GX 301−2

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: suleimanov@astro.uni-tuebingen.de
² Department of Physics and Astronomy, 20014 University of Turku, Finland
³ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Institut de Recherche en Astrophysique et Planétologie, UPS-OMP, CNRS, CNES, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁵ University of British Columbia, Vancouver, BC, V6T 1Z4 Canada
⁶ Space Research Institute (IKI) of Russian Academy of Sciences, Prosoyuznaya ul 84/32, 117997 Moscow, Russian Federation
⁷ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
⁸ Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH UK
⁹ National Research University Higher School of Economics, Faculty of Physics, 101000 Myasnitskaya ul. 20, Moscow, Russia
¹⁰ Instituto de Astrofísicade Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹¹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
¹² Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹³ INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
¹⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹⁵ Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹⁶ NASA Marshall Space Flight Center, Huntsville, AL, 35812 USA
¹⁷ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
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, California, 94305 USA
²⁷ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²⁸ Astronomical Institute of the Czech Academy of Sciences, Bočnń II 1401/1, 14100 Praha 4, Czech Republic
²⁹ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama, 351-0198 Japan
³⁰ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA, 91125 USA
³¹ Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi, 990-8560 Japan
³² Osaka University, 1-1 Yamadaoka, Suita, Osaka, 565-0871 Japan
³³ International Center for Hadron Astrophysics, Chiba University, Chiba, 263-8522 Japan
³⁴ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA, 02215 USA
³⁵ Department of Astrophysics, St. Petersburg State University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
³⁶ Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH, 03824 USA
³⁷ Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO, 63130 USA
³⁸ Finnish Centre for Astronomy with ESO, 20014 University of Turku, Finland
³⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Strada Comunale Cinthia, 80126 Napoli, Italy
⁴⁰ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁴¹ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139 USA
⁴² Graduate School of Science, Division of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8602 Japan
⁴³ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526 Japan
⁴⁴ University of Maryland, Baltimore County, Baltimore, MD, 21250 USA
⁴⁵ NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA
⁴⁶ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD, 20771 USA
⁴⁷ Department of Physics, University of Hong Kong, Pokfulam, Hong Kong
⁴⁸ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA, 16801 USA
⁴⁹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA, 02138 USA
⁵⁰ INAF Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁵¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁵² Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵³ Department of Astronomy, University of Maryland, College Park, Maryland, 20742 USA
⁵⁴ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT UK
⁵⁵ Anton Pannekoek Institute for Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
⁵⁶ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning, 530004 PR China

Received: 24 May 2023
Accepted: 3 August 2023

Abstract

The phase- and energy-resolved polarization measurements of accreting X-ray pulsars (XRPs) allow us to test different theoretical models of their emission, and they also provide an avenue to determine the emission region geometry. We present the results of the observations of the XRP GX 301−2 performed with the Imaging X-ray Polarimetry Explorer (IXPE). A persistent XRP, GX 301−2 has one of the longest spin periods known: ∼680 s. A massive hyper-giant companion star Wray 977 supplies mass to the neutron star via powerful stellar winds. We did not detect significant polarization in the phase-averaged data when using spectro-polarimetric analysis, with the upper limit on the polarization degree (PD) of 2.3% (99% confidence level). Using the phase-resolved spectro-polarimetric analysis, we obtained a significant detection of polarization (above 99% confidence level) in two out of nine phase bins and a marginal detection in three bins, with a PD ranging between ∼3% and ∼10% and a polarization angle varying in a very wide range from ∼0° to ∼160°. Using the rotating vector model, we obtained constraints on the pulsar geometry using both phase-binned and unbinned analyses, finding excellent agreement. Finally, we discuss possible reasons for a low observed polarization in GX 301−2.