Discovery of a variable energy-dependent X-ray polarization in the accreting neutron star GX 5−1

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: sergio.fabiani@inaf.it
² Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, Emilio Segrè, via Archirafi 36, 90123 Palermo, Italy
³ Department of Physics and Astronomy, 20014 University of Turku, Finland
⁴ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 101, 40129 Bologna, Italy
⁶ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
⁷ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
⁸ Center for Astro, Particle and Planetary Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁰ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹¹ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹² INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153, 90146 Palermo, Italy
¹³ Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, 1290 Versoix, Geneva, Switzerland
¹⁴ Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, Wales, UK
¹⁵ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁶ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
¹⁷ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
¹⁸ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
¹⁹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato 09042, Italy
²⁰ INFN, Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato, CA, Italy
²¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
²² Instituto de Astrofísicade Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
²³ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
²⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
²⁵ Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
²⁶ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
²⁷ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
²⁸ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
²⁹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
³⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
³¹ Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
³² Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA
³³ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
³⁴ Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, California 94305, USA
³⁵ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³⁶ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
³⁷ California Institute of Technology, Pasadena, CA 91125, USA
³⁸ Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
³⁹ Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
⁴⁰ University of British Columbia, Vancouver, BC V6T 1Z4, Canada
⁴¹ International Center for Hadron Astrophysics, Chiba University, Chiba 263-8522, Japan
⁴² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁴³ Department of Astrophysics, St. Petersburg State University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
⁴⁴ Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
⁴⁵ Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
⁴⁶ Finnish Centre for Astronomy with ESO, 20014 University of Turku, Finland
⁴⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Strada Comunale Cinthia, 80126 Napoli, Italy
⁴⁸ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁴⁹ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁵⁰ Graduate School of Science, Division of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
⁵¹ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁵² University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁵³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵⁴ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA
⁵⁵ Department of Physics, University of Hong Kong, Pokfulam, Hong Kong
⁵⁶ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16801, USA
⁵⁷ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁵⁹ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁶⁰ Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
⁶¹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁶² Anton Pannekoek Institute for Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

Received: 5 July 2023
Accepted: 21 November 2023

Abstract

We report on the coordinated observations of the neutron star low-mass X-ray binary (NS-LMXB) GX 5−1 in X-rays (IXPE, NICER, NuSTAR, and INTEGRAL), optical (REM and LCO), near-infrared (REM), mid-infrared (VLT VISIR), and radio (ATCA). This Z-source was observed by IXPE twice in March–April 2023 (Obs. 1 and 2). In the radio band the source was detected, but only upper limits to the linear polarization were obtained at a 3σ level of 6.1% at 5.5 GHz and 5.9% at 9 GHz in Obs. 1 and 12.5% at 5.5 GHz and 20% at 9 GHz in Obs. 2. The mid-IR, near-IR, and optical observations suggest the presence of a compact jet that peaks in the mid- or far-IR. The X-ray polarization degree was found to be 3.7%±0.4% (at 90% confidence level) during Obs. 1 when the source was in the horizontal branch of the Z-track and 1.8%±0.4% during Obs. 2 when the source was in the normal-flaring branch. These results confirm the variation in polarization degree as a function of the position of the source in the color-color diagram, as for previously observed Z-track sources (Cyg X-2 and XTE 1701−462). Evidence of a variation in the polarization angle of ∼20° with energy is found in both observations, likely related to the different, nonorthogonal polarization angles of the disk and Comptonization components, which peak at different energies.