Letter to the Editor

Recovery of the X-ray polarisation of Swift J1727.8−1613 after the soft-to-hard spectral transition

¹ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
e-mail: jakub.podgorny@asu.cas.cz
² Department of Physics and Astronomy, 20014 University of Turku, Finland
³ Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, 10691 Stockholm, Sweden
⁴ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
⁵ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁶ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁷ School of Mathematics, Statistics, and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
⁸ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁹ INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, (CA), Italy
¹⁰ Physics Department, McDonnell Center for the Space Sciences, and Center for Quantum Leaps, Washington University in St. Louis, St. Louis, MO 63130, USA
¹¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹² INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153, 90146 Palermo, Italy
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ Anton Pannekoek Institute for Astronomy, Amsterdam, Science Park 904, 1098 NH, The Netherlands
¹⁵ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague 8, Czech Republic
¹⁶ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹⁷ X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁸ California Institute of Technology, Pasadena, CA 91125, USA
¹⁹ Agenzia Spaziale Italiana, Via della Scienza 5, 09047 Selargius (CA), Italy
²⁰ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
²¹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
²² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
²³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
²⁴ Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
²⁵ Physics Department, Tor Vergata University of Rome, Via della Ricerca Scientifica 1, 00133 Rome, Italy
²⁶ INFN – Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
²⁷ Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli, Taoyuan 32001, Taiwan
²⁸ Laboratory for Space Research, The University of Hong Kong, Cyberport 4, Hong Kong
²⁹ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, 100101 Beijing, PR China
³⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands

Received: 30 April 2024
Accepted: 22 May 2024

Abstract

We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8−1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2–8 keV polarisation degree of (3.3 ± 0.4)% and a corresponding polarisation angle of 3° ±4°, which matches the polarisation detected during the rising stage of the outburst, in September–October 2023, within 1σ uncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude.