11institutetext:  INAF Osservatorio Astronomico di Cagliari, Via della Scienza
5, 09047 Selargius (CA), Italy 11email<EMAIL_ADDRESS>22institutetext: Dipartimento di Matematica e Fisica, Università degli Studi
Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy 33institutetext: INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy 44institutetext: Department of Physics and Astronomy,
FI-20014 University of Turku, Finland 55institutetext: INAF Osservatorio di
Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 101, I-40129
Bologna, Italy 66institutetext: INAF IASF-Milano, Via Alfonso Corti 12,
I-20133 Milano, Italy 77institutetext: School of Mathematics, Statistics, and
Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
88institutetext: Serco for the European Space Agency (ESA), European Space
Astronomy Centre, Camino Bajo del Castillo s/n, E-28692 Villanueva de la
Cañada, Madrid, Spain 99institutetext: International Space Science Institute
(ISSI), Hallerstrasse 6, 3012 Bern, Switzerland 1010institutetext: Mullard
Space Science Laboratory, University College London, Holmbury St Mary,
Dorking, Surrey RH5 6NT, UK 1111institutetext: Instituto de Astrofísicade
Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
1212institutetext: INAF Osservatorio Astronomico di Roma, Via Frascati 33,
00040 Monte Porzio Catone (RM), Italy 1313institutetext: Space Science Data
Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
1414institutetext: Istituto Nazionale di Fisica Nucleare, Sezione di Pisa,
Largo B. Pontecorvo 3, 56127 Pisa, Italy 1515institutetext: Dipartimento di
Fisica, Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
1616institutetext: NASA Marshall Space Flight Center, Huntsville, AL 35812,
USA 1717institutetext: Istituto Nazionale di Fisica Nucleare, Sezione di
Torino, Via Pietro Giuria 1, 10125 Torino, Italy 1818institutetext:
Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1,
10125 Torino, Italy 1919institutetext: INAF Osservatorio Astrofisico di
Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy 2020institutetext:
Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via
Sansone 1, 50019 Sesto Fiorentino (FI), Italy 2121institutetext: Istituto
Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto
Fiorentino (FI), Italy 2222institutetext: Agenzia Spaziale Italiana, Via del
Politecnico snc, 00133 Roma, Italy 2323institutetext: Science and Technology
Institute, Universities Space Research Association, Huntsville, AL 35805, USA
2424institutetext: Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor
Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
2525institutetext: Department of Physics and Kavli Institute for Particle
Astrophysics and Cosmology, Stanford University, Stanford, California 94305,
USA 2626institutetext: Institut für Astronomie und Astrophysik, Universität
Tübingen, Sand 1, D-72076 Tübingen, Germany 2727institutetext: Astronomical
Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4,
Czech Republic 2828institutetext: RIKEN Cluster for Pioneering Research, 2-1
Hirosawa, Wako, Saitama 351-0198, Japan 2929institutetext: California
Institute of Technology, Pasadena, CA 91125, USA 3030institutetext: Yamagata
University,1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
3131institutetext: Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871,
Japan 3232institutetext: University of British Columbia, Vancouver, BC V6T
1Z4, Canada 3333institutetext: International Center for Hadron Astrophysics,
Chiba University, Chiba 263-8522, Japan 3434institutetext: Institute for
Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA
02215, USA 3535institutetext: Department of Astrophysics, St. Petersburg State
University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
3636institutetext: Department of Physics and Astronomy and Space Science
Center, University of New Hampshire, Durham, NH 03824, USA 3737institutetext:
Physics Department and McDonnell Center for the Space Sciences, Washington
University in St. Louis, St. Louis, MO 63130, USA 3838institutetext: Finnish
Centre for Astronomy with ESO, 20014 University of Turku, Finland
3939institutetext: Istituto Nazionale di Fisica Nucleare, Sezione di Napoli,
Strada Comunale Cinthia, 80126 Napoli, Italy 4040institutetext: Université de
Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000
Strasbourg, France 4141institutetext: MIT Kavli Institute for Astrophysics and
Space Research, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, MA 02139, USA 4242institutetext: Graduate School of
Science, Division of Particle and Astrophysical Science, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan 4343institutetext:
Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1
Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan 4444institutetext:
University of Maryland, Baltimore County, Baltimore, MD 21250, USA
4545institutetext: NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
4646institutetext: Center for Research and Exploration in Space Science and
Technology, NASA/GSFC, Greenbelt, MD 20771, USA 4747institutetext: Department
of Physics, University of Hong Kong, Pokfulam, Hong Kong 4848institutetext:
Department of Astronomy and Astrophysics, Pennsylvania State University,
University Park, PA 16801, USA 4949institutetext: Université Grenoble Alpes,
CNRS, IPAG, 38000 Grenoble, France 5050institutetext: Center for Astrophysics,
Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
5151institutetext: INAF Osservatorio Astronomico di Brera, via E. Bianchi 46,
23807 Merate (LC), Italy 5252institutetext: Dipartimento di Fisica e
Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova,
Italy 5353institutetext: Dipartimento di Fisica, Università degli Studi di
Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
5454institutetext: Department of Astronomy, University of Maryland, College
Park, Maryland 20742, USA 5555institutetext: Anton Pannekoek Institute for
Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH
Amsterdam, The Netherlands 5656institutetext: Guangxi Key Laboratory for
Relativistic Astrophysics, School of Physical Science and Technology, Guangxi
University, Nanning 530004, China

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

Massimo Cocchi Discovery of strongly variable X-ray polarization in the
neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Andrea Gnarini Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Sergio Fabiani Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Francesco Ursini Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Juri Poutanen Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Fiamma Capitanio
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Anna Bobrikova Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 RubenFarinelli Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Adamantia Paizis
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Lara Sidoli Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Alexandra Veledina Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Stefano Bianchi Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Alessandro Di Marco Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Adam Ingram Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Jari J. E.
Kajava Discovery of strongly variable X-ray polarization in the neutron star
low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Fabio La Monaca Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Giorgio Matt Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Christian
Malacaria Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Romana Mikušincová Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 John Rankin Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Silvia Zane
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Iván Agudo Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Lucio A. Antonelli Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Matteo
Bachetti Discovery of strongly variable X-ray polarization in the neutron star
low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Luca Baldini Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Wayne H. Baumgartner Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Ronaldo Bellazzini
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Stephen D. Bongiorno Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Raffaella Bonino
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Alessandro Brez Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Niccolò Bucciantini
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Simone Castellano Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Elisabetta Cavazzuti
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Chien-Ting Chen Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Stefano Ciprini Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Enrico Costa
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Alessandra De Rosa Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Ettore Del Monte
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Laura Di Gesu Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Niccolò Di Lalla Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Immacolata Donnarumma
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Victor Doroshenko Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Michal Dovčiak Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Steven R. Ehlert Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Teruaki Enoto Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Yuri Evangelista
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Riccardo Ferrazzoli Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Javier A. Garcia
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Shuichi Gunji Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Kiyoshi Hayashida DeceasedDiscovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Jeremy Heyl
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Wataru Iwakiri Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Svetlana G. Jorstad Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Philip Kaaret
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Vladimir Karas Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Fabian Kislat Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Takao Kitaguchi Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Jeffery J. Kolodziejczak Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Henric Krawczynski
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Luca Latronico Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Ioannis Liodakis Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Simone
Maldera Discovery of strongly variable X-ray polarization in the neutron star
low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Alberto Manfreda Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Frédéric Marin Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Andrea Marinucci
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Alan P. Marscher Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Herman L. Marshall Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Francesco
Massaro Discovery of strongly variable X-ray polarization in the neutron star
low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Ikuyuki Mitsuishi Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Tsunefumi Mizuno
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Fabio Muleri Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Michela Negro Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Chi-Yung Ng
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Stephen L. O’Dell Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Nicola Omodei Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Chiara Oppedisano Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Alessandro Papitto
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 George G. Pavlov Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Abel L. Peirson Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Matteo Perri
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Melissa Pesce-Rollins Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Pierre-Olivier Petrucci
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Maura Pilia Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Andrea Possenti Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Simonetta Puccetti
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Brian D. Ramsey Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Ajay Ratheesh Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Oliver J. Roberts
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Roger W. Romani Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Carmelo Sgrò Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Patrick Slane Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Paolo Soffitta Discovery of strongly variable X-ray polarization
in the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462 Gloria Spandre Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Douglas A. Swartz
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Toru Tamagawa Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Fabrizio Tavecchio Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Roberto Taverna Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Yuzuru Tawara Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Allyn F. Tennant Discovery of strongly variable
X-ray polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Nicholas E. Thomas
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Francesco Tombesi Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462 Alessio Trois
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Sergey S. Tsygankov Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Roberto Turolla Discovery
of strongly variable X-ray polarization in the neutron star low-mass X-ray
binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Jacco Vink Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462 Martin C. Weisskopf
Discovery of strongly variable X-ray polarization in the neutron star low-mass
X-ray binary transient XTE J1701$-$462Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462 Kinwah Wu Discovery of strongly variable X-ray polarization in the
neutron star low-mass X-ray binary transient XTE J1701$-$462Discovery of
strongly variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462 Fei Xie Discovery of strongly variable X-ray
polarization in the neutron star low-mass X-ray binary transient XTE
J1701$-$462Discovery of strongly variable X-ray polarization in the neutron
star low-mass X-ray binary transient XTE J1701$-$462Discovery of strongly
variable X-ray polarization in the neutron star low-mass X-ray binary
transient XTE J1701$-$462Discovery of strongly variable X-ray polarization in
the neutron star low-mass X-ray binary transient XTE J1701$-$462

(Received 28 February 2023; accepted May 15 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 $(4.6\pm 0.4)\%$ was measured, the highest value found
up to now for this class of sources. Conversely, only a $\sim$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.

###### Key Words.:

accretion, accretion disks – neutron stars – polarization – X-rays: general –
X-rays: binaries – X-rays: individual: XTE J1701$-$462

††offprints: M. Cocchi

##  1 Introduction

The successful launch in December 2021 of the Imaging X-ray Polarimetry
Explorer (IXPE, Weisskopf et al. 2022; Soffitta et al. 2021) opened a new
investigation territory for the astrophysics of several classes of X-ray
astronomical objects. In particular, three persistent neutron star low-mass
X-ray binaries (NS-LMXBs) were already targeted by IXPE during its 2022
campaign, namely, the Z-class source Cyg X-2 and two bright soft-state Atoll-
sources, GS 1826$-$238 and GX 9$+$9\. These classes are dubbed after the shape
of their X-ray observed hard color versus soft color diagrams (CCDs, see,
e.g., Hasinger & van der Klis, 1989; van der Klis, 1995). Cyg X-2 (Farinelli
et al., 2023) and GX 9+9 (Chatterjee et al., 2023; Ursini et al., 2023) have
shown a significant linearly polarized component (with the polarization
degree, PD, of $\sim 2$%).

Figure 1: 2022 (black) and 2006 (red) outburst light curves of XTE J1701-462.
The starting time $t_{0}$ are MJD 53731.5 and MJD 59824.5 for 2006 and 2022,
respectively. In the instruments’ full energy range, 1 Crab is 75 c s-1 for
RXTE/ASM and 3.8 c s-1 for MAXI. The blue ticks on the abscissa indicate the
epochs of the two IXPE observations (no MAXI data exist during Obs. 1).

XTE J1701$-$462 can be regarded as a rather unique object and is the first
transient NS-LMXB to be observed by IXPE’s modern, sensitive X-ray
polarimeters. Discovered at the beginning of its 2006 outburst (Remillard et
al., 2006), the source was soon identified as the seventh Z-source, the first
one transient in its nature (Homan et al., 2006a, 2007b). It also showed
kilohertz quasi-periodic oscillations in the RXTE/PCA data (Homan et al.,
2006b, 2010; Sanna et al., 2010) during its Z and atoll (lower banana) phases.
A variable radio counterpart to the X-ray source was identified by Fender et
al. (2007) along with a very large-scale jet-like multi-blob structure,
$\sim$2′–3′ south of the central object, first considered to be connected to a
past, unobserved, activity of XTE J1701$-$462, but recently considered
unrelated to the source (Gasealahwe et al., 2023). The 2006 outburst lasted
for a remarkably long time of $\sim 600$ d (Lin et al., 2009), showing an
irregular light curve, especially during the long decaying phase (Šimon,
2015).

The peculiarity of XTE J1701$-$462 is characterized by the fact that is has
experienced all the known spectral states of the NS-LMXB during the 2006
outburst (Lin et al., 2009). In the high-soft state (HSS), it was observed in
all three Z-branches (horizontal, normal, and flaring branches: HB, NB, and
FB). Moreover, the source exhibited the typical transient hard tail (THT)
usually observed in the HB of the Z-sources (Paizis et al., 2006). Then, XTE
J1701$-$462 evolved (Homan et al., 2007a) into a bright soft Atoll source in
the Banana spectral state and finally transitioned into a low-hard Atoll
Island state. Up until now, XTE J1701$-$462 is the only source to have
displayed the NS-LMXB complete spectral evolution, also showing the THT
spectral component. This has enabled studies of the physics of different
accretion states using timing and spectroscopy in an object-unbiased manner.

Though the NS nature of the compact object in XTE J1701$-$462 was already well
established, the detection of two type-I X-ray bursts (Lewin et al., 1993) at
the very end of the outburst (Lin et al., 2009) when the source was in its
Island state, was the most solid confirmation. The bursts had radius expansion
characteristics (Lin et al., 2007) and led to the distance determination of
$\sim 8.8$ kpc, using the burst peak luminosity as a standard candle (Kuulkers
et al., 2003).

A couple of years after its return to quiescence, renewed activity of XTE
J1701$-$462 was claimed in 2010 by Krimm et al. (2010) using the Swift/BAT
data, but not confirmed by Homan et al. (2010). XTE J1701$-$462 turned on
again in September 2022 (Iwakiri et al., 2022), about 16 yr after its first
outburst and about nine months after the beginning of the IXPE space
operations. The 2022 outburst looks different in shape and is less luminous
than the 2006 one (see Fig. 1). According to the MAXI light-curve, the source
could have returned to quiescence after $\sim 200$ days. For this second
outburst, the X-ray polarimetry is a new important tool to study this NS-LMXB
in a unprecedented way. In the following sections, we present and discuss the
results of the XTE J1701$-$462 IXPE campaign.

Figure 2: IXPE CCD of XTE J1701$-$462 during Obs. 1 (red points) and Obs. 2
(blue points). Colors darken with increasing time in order to show the trend
of the patterns. Each point corresponds to 500 s integration time.

##  2 IXPE observations and data analysis

The X-ray polarimeter IXPE (Weisskopf et al., 2022; Soffitta et al., 2021)
targeted XTE J1701$-$462 about 23 days after the MAXI observed onset of the
2022 outburst on September 6 (Iwakiri et al., 2022). IXPE provided high-
sensitivity timing, imaging, spectroscopic, and polarimetric of XTE
J1701$-$462 in the 2–8 keV band. Two observations (Obs. 1 and 2) were
performed on 2022 Sept 29-30 and Oct 8-9 with the nominal exposure of 50 ks
each. All the three IXPE detector units (DUs) were fully operational, yielding
net exposures of 47.7 ks (DU1, DU2) and 47.2 ks (DU3) for Obs. 1 and 48.1 ks
(DU1) and 48.0 ks (DU2,3) for Obs. 2.

Data reduction and analysis were performed using the ixpeobssim software
v30.0.1 (Baldini et al., 2022) and heasoft package version 6.31.1 (Nasa
Heasarc, 2014). The data were filtered by the ixpeobssim tools xpselect and
binned with xpbin to produce images and Stokes $I$, $Q,$ and $U$ spectra
suitable for spectro-polarimetric analysis with xspec. We used the latest
(version 12) IXPE response matrices available on the ixpeobssim public
repository.111https://github.com/lucabaldini/ixpeobssim The source region was
selected from the image of each of the three detector units (DU), with the
source centred in a circular region of 60″ in radius. No background
subtraction was applied due to the relatively high count rate of the source
($\sim$30 cts s-1 per DU) (see Di Marco et al., 2023). The data analysis was
performed following the weighted method.222In the weighted analysis method, a
weight is assigned to each photo-electron track depending on its shape.

The normalized Stokes parameters $q=Q/I$ and $u=U/I$, corresponding to a
polarization degree (PD) and angle (PA), as well as their uncertainties were
calculated using the model-independent pcube binning algorithm of ixpeobssim.
On the contrary, PD and PA obtained with xspec require the definition of a
spectro-polarimetric model. The time-resolved spectra were obtained using the
procedure described above and then the spectro-polarimetric analysis was
performed by xspec.

Figure 3: Observed 2–8 keV overall polarization signal (black crosses) and for
the single IXPE DUs (colored crosses) for Obs. 1 and Obs. 2. Stokes parameters
computed by ixpeobsim.  Figure 4: IXPE photon (upper panel) and $EF_{E}$
spectra (lower panel) of XTE J1701$-$462 during Obs. 1, along with the best-
fit model (see Table 1) consisting of diskbb for the accretion disk (blue
dotted curve) and bbodyrad for the BL-SL layer (red dashed) as well as their
sum (black solid). The residuals are shown in the middle panel. Blue, red, and
green color correspond to DUs 1–3, respectively.  Table 1: Spectro-
polarimetric results for Obs. 1 and 2 for a disk+BL model.

Parameters | Obs. 1 | Obs. 2
---|---|---
const*tbabs*(diskbb+bbodyrad)
tbabs |  |
— $N_{\rm H}$ ($10^{22}\leavevmode\nobreak\ {\rm cm}^{-2}$) | $2.7\pm 0.3$ | $3.4_{-0.3}^{+0.5}$
diskbb |  |
— $T_{\rm in}$ (keV) | $0.72_{-0.07}^{+0.09}$ | $0.57\pm 0.04$
— $R_{\rm in}\ \sqrt{\cos i}$ (km)aa$a$Radia are determined for the distance of 10 kpc. | $37_{-12}^{+16}$ | $78_{-23}^{+35}$
bbodyrad |  |
— $T$ (keV) | $1.31_{-0.02}^{+0.03}$ | $1.13\pm 0.01$
— $R_{\rm bb}$ (km) | $18\pm 3$ | $26_{-3}^{+4}$
const |  |
— DU1 | $[1]$ | $[1]$
— DU2 | $0.981\pm 0.003$ | $0.967\pm 0.003$
— DU3 | $0.917\pm 0.003$ | $0.950\pm 0.004$
Reduced $\chi^{2}$ (d.o.f) | 1.044 (440) | 1.080 (422)
Flux ($10^{-9}$ erg cm-2 s-1) | $8.34_{-0.62}^{+0.01}$ | $8.73_{-0.54}^{+0.02}$
Photon flux (cm-2 s-1) | $1.304_{-0.125}^{+0.001}$ | $1.417_{-0.114}^{+0.004}$
disk photonsbb$b$Fraction of the total photon flux. | 0.247 | 0.226
polconst*(diskbb+bbodyrad)cc$c$Spectroscopic parameters were first set to
their best-fit values (upper panel) and then allowed to vary in the spectro-
polarimetric fit.
— PD (%) | $4.58\pm 0.37$ | $0.65\pm 0.35$
— PA (deg) | $-37.7\pm 2.3$ | $-57\pm 16$
(polconst*diskbb + polconst*bbodyrad)dd$d$The disk PA is set $90\degr$ apart
from the BL one.
diskbb |  |
— PD (%) | $<1.6$ | $<3.2$
— PA (deg) | [PABL+90] | [PABL+90]
bbodyrad (for BL) |  |
— PD (%) | $6.14_{-0.49}^{+0.69}$ | $1.22\pm 0.65$
— PA (deg) | $-38.3\pm 2.3$ | $-54\pm 14$
bbodyradee$e$As above, but freezing [PD=0] for the disk component. |  |
— PD (%) | $6.16_{-0.50}^{+0.52}$ | $0.90\pm 0.45$
— PA (deg) | $-38.3\pm 2.3$ | $-56\pm 15$
333DU energy range is 2–8 keV. The spectral and polarimetric fits errors and
upper limits are joint computed at $2\sigma$ (95% confidence). A systematics
of 1% is applied in xspec in order to account for slight instrumental
miscalibration. Normalizations of DU2 and DU3 are relative to DU1. Frozen or
linked parameters are in square brackets.

##  3 Results

We constructed the time-resolved CCD for both observations (see Fig. 2).
During Obs. 1, the source moved from a lower HB to an upper HB diagram
position. This coincides with the 2-8 keV time profile flux decreasing by
$\sim 28\%$, in an anti-correlation to its 4–8/2–4 keV X-ray hardness ratio,
as is typical for a Z-source in the HB. Furthermore,Obs. 2 found the source in
its NB, moving from the upper NB to the lower NB. During the observation, the
flux and the hardness ratio were stable within a few percent, in agreement
with a NB where the source is expected to be very close to its Eddington-
limited luminosity.

Figure 5: Average polarization for Obs. 1 and 2 computed via spectro-
polarimetric analysis using a single polconst convolving component in xspec
(see Table 1).

The IXPE polarimetric results obtained by ixpeobsim showed a (model-
independent) strong variability of the linear PD between the two observations
(see Fig. 3). An average PD of $(4.6\pm 0.4)\%$ was found during Obs. 1, while
in Obs. 2 ($\sim 10$ d after Obs. 1), the 2–8 keV polarimetric signal could
not be constrained with the 99% upper limit of 1.5%.

We described the time averaged spectra of the two observations by a standard
two-component HSS model consisting of a multicolor disk and a harder boundary-
spreading layer (BL-SL) emission (e.g., Popham & Sunyaev, 2001; Revnivtsev et
al., 2013). The BL emission can be modeled in xspec either using a pure
blackbody (bbodyrad), or an unsaturated Comptonization model, such as comptt
(Titarchuk, 1994). The wide-band measurements (e.g. with BeppoSAX) suggest a
Comptonized emission from a BL (e.g. Di Salvo et al., 2002; Farinelli et al.,
2009) as a better way to describe the data. However, the IXPE spectra of XTE
J1701$-$462 turned out difficult to fit by a comptt component without
additional assumptions, due to a degeneracy in the parameter space. This is
most likely due to the relatively narrow data pass-band, which prevents the
energy cutoff from being constrained. Therefore, we considered replacing in
the fits the Comptonization model by a pure blackbody component. In this case,
the fits converged without further constraints.

The disk emission was modeled by diskbb and diskpn (Mitsuda et al., 1984;
Gierliński et al., 1999). Due to the increased parameter space for diskpn, the
inner radius resulted unconstrained, so it could not be distinguished from
diskbb, and, in fact, the temperatures of the two models are consistent with
each other.

A tbabs (Wilms et al., 2000) multiplicative component was included to account
for the low-energy absorption due to the interstellar medium; abundances and
cross-section tables according to Asplund et al. (2009) and Verner et al.
(1996), (aspl, vern in xspec). We also included a normalizing cross-
calibration multiplicative factor to account for the uncertainties in the
effective areas of three telescopes. The resulting best fits for both
observations are reported in Table 1 (upper panel) for the disk plus the
blackbody-modeled BL. Photon and energy fluxes are slightly higher in Obs. 2,
in agreement with an expected larger energy production in the NB. The BL and
the inner disk are found to be hotter in Obs. 1, while the inner disk radius
seems larger in Obs. 2. The opposite occurs for the BL sphere-equivalent
radius. We tested the reasonable hypothesis that $N_{\rm H}$ and the
normalizing factors were the same for both observations. Nevertheless, the
corresponding fits could be accepted only by relaxing the systematics to 1.3%
and 1.5%. On the other hand, the interpolated value (nh ftool, v.3) of $N_{\rm
H}$ at the source coordinates is $\sim 0.6\times 10^{22}{\rm cm}^{-2}$,
namely, it is several times lower than those presented in Lin et al. (2009)
and in the present work. This suggests the source may be intrinsically (and
possibly variably) absorbed by nearby cold structures.

The Obs. 1 spectral best fit is shown in Fig. 4. The softer disk component
dominates in photons (upper panel) up to $\sim 2.5$ keV, being still important
up to $\sim 3.5$ keV. In the 2–8 keV band, the disk photons account for $\sim
24\%$ of the total. The harder BL completely dominates the energy budget
(lower panel, $EF_{E}$ representation).

Figure 6: Polarization contours in Obs. 1 for the diskbb and bbodyrad
components estimated with polconst (see also Table 1, lower fit).

Concerning the average polarization in the two observations, fully ixpeobsim-
compatible results were achieved by the spectral-polarimetric data fits using
xspec (please compare Fig. 5 to Fig. 3), where the energy-independent
polarized multiplicative model component (polconst) was used (see Table 1).

Motivated by the spectral decomposition shown in Fig. 4, we performed a
spectro-polarimetric analysis, assuming that the two components may have
different polarization properties. In Table 1, lower panel, the average and
disk-versus-BL resolved (the latter obtained by applying a dedicated polconst
to each of the main spectral components) polarization results are reported.
The BL-related signals were constrained in both observations, with a
polarization degree of 6.1% and 1.2%, respectively. In Obs. 2 the signal is
incompatible with null polarization with a $>$99.7% probability. The best fits
and errors were obtained by joint variation of the spectral and polarimetric
parameters (Rawat et al., 2023) starting from the best fit to the spectra
shown in the upper part of Table 1. As in Farinelli et al. (2023), we assumed
a 90° separation of the disk polarization angle (PA) with respect to the BL
one. This is expected in the absence of strong GR effects and because of the
different geometries of the scattered BL radiation and the intrinsically
polarized disk radiation (e.g., Chandrasekhar (1960)). We also investigated
the possibility of fully independent PAs for the two components, but no
conclusion could be drawn because of the unconstrained and not statistically
significant disk signal (see also Fig. 6 for Obs. 1).

The strong average polarization signal during Obs. 1 does not show a clear
evidence for time variability during the measurement, although the
polarization time profile (obtained in five 10 ks time intervals) may suggest
a slightly lower average signal in the first time bin (see Fig. 7). The energy
dependence of the polarization during both observations is not evident either
(as shown in Fig. 8). Nevertheless, the lowest energy (2–3 keV) bin in Obs. 1
shows a lower polarization at a 90% confidence level with respect to higher
energy bins.

In Obs. 2, only the 5–8 keV energy bin is compatible with a non-null
polarization, at a confidence of 99.73% . If we assume Obs. 2 is unpolarized
in all the four independent energy bins, the probability of claiming a false
polarization for the 5–8 keV bin is $1-(1-p)^{n}$, where $p=0.27\%$ (for the
single-bin confidence level of 99.73%) and $n=4$. This probability is
$1.08\%$. The 5-8 keV polarization, being roughly aligned with the signal of
Obs. 1, supports the possibility of a (weak) BL-related signal in Obs. 2 as
well.

Figure 7: Time-binned polarization of Obs. 1 at 90% confidence level. Zero
polarization is excluded for all time bins at 99.9% confidence level.

Figure 8: 90% confidence level contours for the PD and PA in four IXPE energy
bands (2–3, 3–4, 4–5, and 5–8 keV) for Obs. 1 (left panel) and Obs. 2 (right
panel).

##  4 Discussion

The results in Section 3 show that XTE J1701$-$462 was observed by IXPE during
its outburst Z evolutionary phase, in agreement with the reports of other
X-ray observatories (e.g., Thomas et al., 2022). The IXPE light curve, CCD
diagrams, and spectroscopic analysis confirm this result with a high level of
confidence. In this context, the discovery of a strong polarimetric signal
during the 2022 Sept 29 observation (Obs. 1) poses a new important constraint
on the physical models and the geometry of the emission region of Z-sources.

The relative fluxes of the disk and the BL during Obs. 1 (Fig. 4) are in
agreement with those typically observed for Z-sources in their HB CCD track
(Farinelli et al., 2009). Also, the CCD time evolution (Fig. 2) during the
observation indicates that the source was in a HB of the Z-track. Conversely,
according to the CCD and the average spectral characteristics, Obs. 2 most
likely found the source in the NB of the Z-track, with no (or relatively
insignificant) excursions to the HB; in this case, only a $\sim 1\%$ polarized
signal that is strictly connected to the BL component could be established.
This indicates that polarization is connected to the source position along its
Z-track, and it is strongest in the upper-intermediate HB.

Taking these findings into account, it is necessary to explain the evidence
for strong (possibly transient) polarization in the upper HB of the Z-track.
Actually, the measured PD of $\sim 6\%$ associated with the BL (direct and/or
disk-reflected) emission as observed from a Z-source in a HB is rather
challenging to explain.

An electron-scattering dominated accretion disk can be polarized at a level of
4% (Chandrasekhar, 1960) for the inclination of $i=75\degr$ as inferred for
XTE J1701$-$462 by Lin et al. (2009). However, the disk dominates below $\sim
3$ keV and so, it cannot be responsible for the polarized emission at higher
energies where the BL dominates. The BL itself, as a geometrically thicker
extension of the inner accretion disk (i.e. lying in the same plane) is not
likely to be the source of the polarized signal. Achieving PD of 5% requires a
high inclination $>78\degr$ (Chandrasekhar, 1960), typical for a dipper, while
XTE J1701$-$462 does not show dips. Moreover, in a very similar source, Cyg
X-2, polarization is parallel to the jet direction (Farinelli et al., 2023),
while the BL model would predict polarization perpendicular to the jet. A high
polarization of $\sim 10\%$ can be easily achieved by multiple Compton
scatterings in an optically thin, plane-parallel hot corona above the disk
(Poutanen & Svensson, 1996), however, this spectral component is not observed
in the IXPE data.

The spreading layer (SL) (Inogamov & Sunyaev, 1999), namely, the BL at the NS
surface, can potentially be polarized (Lapidus & Sunyaev, 1985). We have
computed the expected PD for this model assuming that the local emission
corresponds to the semi-infinite electron-scattering dominated atmosphere
(Chandrasekhar, 1960). We considered layers of different latitudinal extents,
narrow belts, and fully covered NS, and we accounted for the relativistic
effects related to the rotation of the polarization plane due to the fast
motion of the SL (Poutanen, 2020). We did not obtain a PD exceeding $\sim
1.5\%$ in any of these cases. Thus, we can conclude that the SL alone is not
responsible for the observed polarization.

However, radiation that is emitted by the SL at the NS surface may be
scattered by the surrounding material. If the SL does not extend to very high
latitudes, then reflection from the ionized inner accretion disk could produce
PD up to 6% in the total spectrum (see Fig. 7 in Lapidus & Sunyaev, 1985)
parallel to the rotation axis, when neglecting the inner disk direct emission
(which is most likely polarized, but counteracting as being perpendicular to
the axis). However, it is also important to note that disk polarization drops
very rapidly with decreasing inclination, while the PD of the reflected signal
has a much slower dependence on inclination. This implies that the PD of the
total signal will be somewhat smaller at lower energies due to the dilution by
nearly unpolarized disk radiation, but will not drop to zero due to
cancellation of perpendicular polarization.

Radiation from the SL can also be scattered in a wind from the disk – or a
jet, if one is present in the system. Radio loudness is mostly reported when
the Z-sources are found in their HB phase (e.g., Done et al., 2007). Moreover,
the observations of transient hard tails (THT) in the spectra of Z-sources are
strictly correlated to HB positions in their CCDs (D’Amico et al., 2001; Di
Salvo et al., 2002; Iaria et al., 2004; Farinelli et al., 2005) and possibly
produced by scattering in a jet-like outflow (Reig & Kylafis, 2016).

It is thus tempting to link the HB strong polarimetric signal to the
accelerating mechanism (still not fully understood) that produces the jet
emission and the possibly jet-related THT (Reig & Kylafis, 2016). However, the
contribution of the THT to the flux in the IXPE band is not expected to exceed
10% (see, e.g., Paizis et al., 2006) and therefore it is not very likely that
this component is responsible for a strong polarized signal, unless the BL
photons are single-scattered in the Thomson regime. Furthermore, it is not
clear why the PD would then drop at lower energies, unless the spectrum of the
THT has a low-energy cutoff around 3-4 keV; however, this can be nevertheless
expected because a power-law component with no low energy cutoff would cause a
low-energy fit contamination, which is not observed in the wide band spectra
of Z-sources during their THT spectral phase (Paizis et al., 2006; Farinelli
et al., 2009).

As the previously claimed extended jet-like structure to the south of XTE
J1701$-$462 (Fender et al., 2007) is more likely a background feature
(Gasealahwe et al., 2023), there are no convincing arguments as to why the jet
would be the source of polarized signal. The central source itself, however,
was radio loud (Fender et al., 2007), implying the existence of some outflow
from the system. Also, the similarity of XTE J1701$-$462 to the prototype Cyg
X-2 suggests that polarimetric signal is likely parallel to the outflow. The
scattering of central radiation in the outflow can produce polarization. If
the outflow occupied a large area above the disk, both the emission of the SL
and the disk should be polarized in a similar fashion, which is not the case.

On the other hand, the outflow originating from the inner part of the
accretion disk may face much larger optical depth along the disk plane than
perpendicular to it. Thus, the disk radiation which is beamed strongly along
its normal will be affected very little, whereas the SL radiation, which is
emitted more isotropically (or even slightly beamed along the disk surface)
can be scattered much more efficiently. Radiation injected along a slab and
Thomson scattered there gets polarized parallel to the slab normal with the
PD$\,=\sin^{2}i/(3-\cos^{2}i)$ (Sunyaev & Titarchuk, 1985), reaching the
maximum of 33% at 90° inclination. The PD in this case has a rather weak
dependence on the inclination and even at $i=70\degr$, the PD is still $\sim
30\%$. The observed 5–6% polarization requires that about 20% of the SL
radiation is scattered in the wind, which does not appear to be implausible.

Finally, the question of why the PD is much smaller in Obs. 2 must be
addressed. If scattering in the wind is responsible for the high PD in Obs. 1,
then the wind has to almost disappear – or, at least, its optical depth should
become smaller in the NB. If reflection from the disk is the source of the
high PD, the most straightforward explanation would be that the latitudinal
extent of the SL has decreased during Obs. 2, so that it no longer illuminates
the disk efficiently.

Our IXPE data (see Table 1) also suggest that the inner disk could be farther
from the SL during Obs. 2, possibly due to very high radiation pressure
(Z-sources reach their Eddington limit when in the NB). This fact also points
to less efficient disk illumination. Of course, all these speculations need
the support of reliable simulations and this will likely be the subject of
future investigations.

##  5 Summary

We reported a $\sim 5\%$ linearly polarized signal in IXPE observations of the
NS transient Z-source XTE J1701$-$462\. The signal is found when the source is
at the upper-and-intermediate horizontal branch position in the CCD diagram.
The unprecedentedly high strength of the signal for this class of compact
objects can hardly be explained by a standard boundary layer plus accretion
disk scenario. We speculate that scattering of the boundary-spreading layer
radiation by either the accretion disk, a corona above the disk, or an
optically thin wind can be responsible for the observed polarimetric
characteristics. On the other hand, the polarimetric signal was much lower
when the source was in the normal branch, implying a change in the accretion
geometry or the wind properties.

###### Acknowledgements.

The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian
mission. The US contribution is supported by the National Aeronautics and
Space Administration (NASA) and led and managed by its Marshall Space Flight
Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The
Italian contribution is supported by the Italian Space Agency (Agenzia
Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-
INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center
(SSDC) with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and by
the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di
Fisica Nucleare (INFN) in Italy. This research used data products provided by
the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with additional
software tools by the High-Energy Astrophysics Science Archive Research Center
(HEASARC), at NASA Goddard Space Flight Center (GSFC). J.P. and A.V.
acknowledge support from the Academy of Finland grant 333112. A.B. is
supported by the Finnish Cultural Foundation grant 00220175. The authors also
thank the anonymous referee for the useful comments and suggestions.

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