Letter to the Editor

Discovery of strongly variable X-ray polarization in the neutron star low-mass X-ray binary transient XTE J1701−462

¹ INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
e-mail: massimo.cocchi@inaf.it
² Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
³ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁵ INAF Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 101, 40129 Bologna, Italy
⁶ INAF IASF-Milano, Via Alfonso Corti 12, 20133 Milano, Italy
⁷ School of Mathematics, Statistics, and Physics, Newcastle University, Newcastle upon Tyne, NE1 7RU UK
⁸ Serco for the European Space Agency (ESA), European Space Astronomy Centre, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
¹⁰ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT UK
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 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 Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
¹⁸ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
¹⁹ INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
²⁰ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino, FI, Italy
²¹ Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino, (FI), Italy
²² Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
²³ Science and Technology Institute, Universities Space Research Association, Huntsville, AL, 35805 USA
²⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
²⁵ Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA, 94305 USA
²⁶ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁷ Astronomical Institute of the Czech Academy of Sciences, Boní II 1401/1, 14100 Praha 4, Czech Republic
²⁸ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama, 351-0198 Japan
²⁹ California Institute of Technology, Pasadena, CA, 91125 USA
³⁰ Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi, 990-8560 Japan
³¹ Osaka University, 1-1 Yamadaoka, Suita, Osaka, 565-0871 Japan
³² University of British Columbia, Vancouver, BC, V6T 1Z4 Canada
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 Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵⁰ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA, 02138 USA
⁵¹ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
⁵² Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁵³ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵⁴ Department of Astronomy, University of Maryland, College Park, MD, 20742 USA
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: 28 February 2023
Accepted: 15 May 2023

Abstract

Context. After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701−462 turned on again in September 2022, allowing for the first study of its X-ray polarimetric characteristics by a dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE).

Aims. Polarimetric studies of XTE J1701−462 have been expected to improve our understanding of accreting weakly magnetized neutron stars, in particular, the physics and the geometry of the hot inner regions close to the compact object.

Methods. The IXPE data of two triggered observations were analyzed using time-resolved spectroscopic and polarimetric techniques, following the source along its Z-track of the color–color diagram.

Results. During the first pointing on 2022 September 29, an average 2–8 keV polarization degree of (4.6 ± 0.4)% was measured, the highest value found up to now for this class of sources. Conversely, only a ∼0.6% average degree was obtained during the second pointing ten days later.

Conclusions. The polarimetric signal appears to be strictly related to the higher energy blackbody component associated with the boundary layer (BL) emission and its reflection from the inner accretion disk, and it is as strong as 6.1% and 1.2% (> 95% significant) above 3–4 keV for the two measurements, respectively. The variable polarimetric signal is apparently related to the spectral characteristics of XTE J1701−462, which is the strongest when the source was in the horizontal branch of its Z-track and the weakest in the normal branch. These IXPE results provide new important observational constraints on the physical models and geometry of the Z-sources. Here, we discuss the possible reasons for the presence of strong and variable polarization among these sources.