# Discovery of X-ray Polarization from the Black Hole Transient Swift
J1727.8$–$1613

Alexandra Veledina Department of Physics and Astronomy, 20014 University of
Turku, Finland  Nordita, KTH Royal Institute of Technology and Stockholm
University, Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden  Fabio Muleri
INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del
Cavaliere 100, 00133 Roma, Italy  Michal Dovčiak Astronomical Institute of the
Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
Juri Poutanen Department of Physics and Astronomy, 20014 University of Turku,
Finland  Ajay Ratheesh INAF Istituto di Astrofisica e Planetologia Spaziali,
Via del Fosso del Cavaliere 100, 00133 Roma, Italy  Fiamma Capitanio INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy  Giorgio Matt Dipartimento di Matematica e Fisica,
Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
Paolo Soffitta INAF Istituto di Astrofisica e Planetologia Spaziali, Via del
Fosso del Cavaliere 100, 00133 Roma, Italy  Allyn F. Tennant NASA Marshall
Space Flight Center, Huntsville, AL 35812, USA  Michela Negro Department of
Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
Philip Kaaret NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
Enrico Costa INAF Istituto di Astrofisica e Planetologia Spaziali, Via del
Fosso del Cavaliere 100, 00133 Roma, Italy  Adam Ingram School of Mathematics,
Statistics, and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
Jiří Svoboda Astronomical Institute of the Czech Academy of Sciences, Boční II
1401/1, 14100 Praha 4, Czech Republic  Henric Krawczynski Physics Department
and McDonnell Center for the Space Sciences, Washington University in St.
Louis, St. Louis, MO 63130, USA  Stefano Bianchi Dipartimento di Matematica e
Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146
Roma, Italy  James F. Steiner Center for Astrophysics, Harvard & Smithsonian,
60 Garden St, Cambridge, MA 02138, USA  Javier A. García X-ray Astrophysics
Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  Vadim
Kravtsov Department of Physics and Astronomy, 20014 University of Turku,
Finland  Anagha P. Nitindala Department of Physics and Astronomy, 20014
University of Turku, Finland  Melissa Ewing School of Mathematics, Statistics,
and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK  Guglielmo
Mastroserio INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5,
09047 Selargius (CA), Italy  Andrea Marinucci Agenzia Spaziale Italiana, Via
del Politecnico snc, 00133 Roma, Italy  Francesco Ursini Dipartimento di
Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale
84, 00146 Roma, Italy  Francesco Tombesi Dipartimento di Fisica, Università
degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133
Roma, Italy  Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor
Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy  Department of
Astronomy, University of Maryland, College Park, Maryland 20742, USA  Sergey
S. Tsygankov Department of Physics and Astronomy, 20014 University of Turku,
Finland  Yi-Jung Yang Department of Physics, The University of Hong Kong,
Pokfulam Rd, Hong Kong  Laboratory for Space Research, The University of Hong
Kong, Cyberport 4, Hong Kong  Martin C. Weisskopf NASA Marshall Space Flight
Center, Huntsville, AL 35812, USA  Sergei A. Trushkin Special Astrophysical
Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, 369167,
Karachayevo-Cherkessia, Russia  Elise Egron INAF Osservatorio Astronomico di
Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy  Maria Noemi
Iacolina Agenzia Spaziale Italiana, via della Scienza 5, 09047, Selargius
(CA), Italy  Maura Pilia INAF Osservatorio Astronomico di Cagliari, Via della
Scienza 5, 09047 Selargius (CA), Italy  Lorenzo Marra Dipartimento di
Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale
84, 00146 Roma, Italy  Romana Mikušincová Dipartimento di Matematica e Fisica,
Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
Edward Nathan Cahill Center for Astronomy and Astrophysics, California
Institute of Technology, Pasadena, CA91125, USA  Maxime Parra Université
Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  Pierre-Olivier Petrucci
Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  Jakub Podgorný
Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR
7550, 67000 Strasbourg, France  Astronomical Institute of the Czech Academy of
Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic  Astronomical
Institute, Charles University, V Holešovičkách 2, CZ-18000, Prague, Czech
Republic  Stefano Tugliani Istituto Nazionale di Fisica Nucleare, Sezione di
Torino, Via Pietro Giuria 1, 10125, Torino, Italy; Dipartimento di Fisica,
Università degli Studi di Torino, Via Pietro Giuria 1, 10125, Torino, Italy
Silvia Zane Mullard Space Science Laboratory, University College London,
Holmbury St Mary, Dorking, Surrey RH5 6NT, UK  Wenda Zhang National
Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road,
Beijing 100101, China  Iván Agudo Instituto de Astrofísicade Andalucía – CSIC,
Glorieta de la Astronomía s/n, 18008 Granada, Spain  Lucio A. Antonelli INAF
Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone
(RM), Italy  Space Science Data Center, Agenzia Spaziale Italiana, Via del
Politecnico snc, 00133 Roma, Italy  Matteo Bachetti INAF Osservatorio
Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
Luca Baldini Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B.
Pontecorvo 3, 56127 Pisa, Italy  Dipartimento di Fisica, Università di Pisa,
Largo B. Pontecorvo 3, 56127 Pisa, Italy  Wayne H. Baumgartner NASA Marshall
Space Flight Center, Huntsville, AL 35812, USA  Ronaldo Bellazzini Istituto
Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127
Pisa, Italy  Stephen D. Bongiorno NASA Marshall Space Flight Center,
Huntsville, AL 35812, USA  Raffaella Bonino Istituto Nazionale di Fisica
Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1,
10125 Torino, Italy  Alessandro Brez Istituto Nazionale di Fisica Nucleare,
Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy  Niccolò Bucciantini
INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze,
Italy  Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze,
Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy  Istituto Nazionale di
Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino
(FI), Italy  Simone Castellano Istituto Nazionale di Fisica Nucleare, Sezione
di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy  Elisabetta Cavazzuti
Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy  Chien-
Ting Chen Science and Technology Institute, Universities Space Research
Association, Huntsville, AL 35805, USA  Stefano Ciprini Istituto Nazionale di
Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica
1, 00133 Roma, Italy  Space Science Data Center, Agenzia Spaziale Italiana,
Via del Politecnico snc, 00133 Roma, Italy  Alessandra De Rosa INAF Istituto
di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133
Roma, Italy  Ettore Del Monte INAF Istituto di Astrofisica e Planetologia
Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy  Laura Di Gesu
Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy  Niccolò
Di Lalla Department of Physics and Kavli Institute for Particle Astrophysics
and Cosmology, Stanford University, Stanford, California 94305, USA
Alessandro Di Marco INAF Istituto di Astrofisica e Planetologia Spaziali, Via
del Fosso del Cavaliere 100, 00133 Roma, Italy  Immacolata Donnarumma Agenzia
Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy  Victor
Doroshenko Institut für Astronomie und Astrophysik, Universität Tübingen, Sand
1, 72076 Tübingen, Germany  Steven R. Ehlert NASA Marshall Space Flight
Center, Huntsville, AL 35812, USA  Teruaki Enoto RIKEN Cluster for Pioneering
Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan  Yuri Evangelista INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy  Sergio Fabiani INAF Istituto di Astrofisica e
Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
Riccardo Ferrazzoli INAF Istituto di Astrofisica e Planetologia Spaziali, Via
del Fosso del Cavaliere 100, 00133 Roma, Italy  Shuichi Gunji Yamagata
University,1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan  Kiyoshi
Hayashida  Deceased  Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871,
Japan  Jeremy Heyl University of British Columbia, Vancouver, BC V6T 1Z4,
Canada  Wataru Iwakiri International Center for Hadron Astrophysics, Chiba
University, Chiba 263-8522, Japan  Svetlana G. Jorstad Institute for
Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA
02215, USA  Department of Astrophysics, St. Petersburg State University,
Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia  Vladimir
Karas Astronomical Institute of the Czech Academy of Sciences, Boční II
1401/1, 14100 Praha 4, Czech Republic  Fabian Kislat Department of Physics and
Astronomy and Space Science Center, University of New Hampshire, Durham, NH
03824, USA  Takao Kitaguchi  RIKEN Cluster for Pioneering Research, 2-1
Hirosawa, Wako, Saitama 351-0198, Japan  Jeffery J. Kolodziejczak NASA
Marshall Space Flight Center, Huntsville, AL 35812, USA  Fabio La Monaca INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy  Luca Latronico Istituto Nazionale di Fisica Nucleare,
Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy  Ioannis Liodakis
NASA Marshall Space Flight Center, Huntsville, AL 35812, USA  Simone Maldera
Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1,
10125 Torino, Italy  Alberto Manfreda Istituto Nazionale di Fisica Nucleare,
Sezione di Napoli, Strada Comunale Cinthia, 80126 Napoli, Italy  Frédéric
Marin Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg,
UMR 7550, 67000 Strasbourg, France  Alan P. Marscher Institute for
Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA
02215, USA  Herman L. Marshall MIT Kavli Institute for Astrophysics and Space
Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, MA 02139, USA  Francesco Massaro Istituto Nazionale di Fisica
Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1,
10125 Torino, Italy  Ikuyuki Mitsuishi  Graduate School of Science, Division
of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-
ku, Nagoya, Aichi 464-8602, Japan  Tsunefumi Mizuno Hiroshima Astrophysical
Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima,
Hiroshima 739-8526, Japan  Chi-Yung Ng Department of Physics, The University
of Hong Kong, Pokfulam Rd, Hong Kong  Stephen L. O’Dell NASA Marshall Space
Flight Center, Huntsville, AL 35812, USA  Nicola Omodei Department of Physics
and Kavli Institute for Particle Astrophysics and Cosmology, Stanford
University, Stanford, California 94305, USA  Chiara Oppedisano Istituto
Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125
Torino, Italy  Alessandro Papitto INAF Osservatorio Astronomico di Roma, Via
Frascati 33, 00040 Monte Porzio Catone (RM), Italy  George G. Pavlov
Department of Astronomy and Astrophysics, Pennsylvania State University,
University Park, PA 16801, USA  Abel L. Peirson Department of Physics and
Kavli Institute for Particle Astrophysics and Cosmology, Stanford University,
Stanford, California 94305, USA  Matteo Perri Space Science Data Center,
Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy  INAF
Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone
(RM), Italy  Melissa Pesce-Rollins Istituto Nazionale di Fisica Nucleare,
Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy  Andrea Possenti
INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047
Selargius (CA), Italy  Simonetta Puccetti Space Science Data Center, Agenzia
Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy  Brian D. Ramsey
NASA Marshall Space Flight Center, Huntsville, AL 35812, USA  John Rankin INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy  Oliver J. Roberts Science and Technology Institute,
Universities Space Research Association, Huntsville, AL 35805, USA  Roger W.
Romani Department of Physics and Kavli Institute for Particle Astrophysics and
Cosmology, Stanford University, Stanford, California 94305, USA  Carmelo Sgrò
Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3,
56127 Pisa, Italy  Patrick Slane Center for Astrophysics, Harvard &
Smithsonian, 60 Garden St, Cambridge, MA 02138, USA  Gloria Spandre Istituto
Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127
Pisa, Italy  Douglas A. Swartz Science and Technology Institute, Universities
Space Research Association, Huntsville, AL 35805, USA  Toru Tamagawa RIKEN
Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Fabrizio Tavecchio INAF Osservatorio Astronomico di Brera, via E. Bianchi 46,
23807 Merate (LC), Italy  Roberto Taverna Dipartimento di Fisica e Astronomia,
Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy  Yuzuru
Tawara  Graduate School of Science, Division of Particle and Astrophysical
Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602,
Japan  Nicholas E. Thomas NASA Marshall Space Flight Center, Huntsville, AL
35812, USA  Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor
Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy  Department of
Astronomy, University of Maryland, College Park, Maryland 20742, USA  Alessio
Trois INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047
Selargius (CA), Italy  Roberto Turolla Dipartimento di Fisica e Astronomia,
Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy  Mullard
Space Science Laboratory, University College London, Holmbury St Mary,
Dorking, Surrey RH5 6NT, UK  Jacco Vink Anton Pannekoek Institute for
Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH
Amsterdam, The Netherlands  Kinwah Wu Mullard Space Science Laboratory,
University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK  Fei
Xie Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical
Science and Technology, Guangxi University, Nanning 530004, China  INAF
Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere
100, 00133 Roma, Italy

###### Abstract

We report the first detection of the X-ray polarization of the bright
transient Swift J1727.8$–$1613 with the Imaging X-ray Polarimetry Explorer.
The observation was performed at the beginning of the 2023 discovery outburst,
when the source resided in the bright hard state. We find a time- and energy-
averaged polarization degree of $4.1\%\pm 0.2\%$ and a polarization angle of
$2\fdg 2\pm 1\fdg 3$ (errors at 68% confidence level; this translates to
$\sim$20$\sigma$ significance of the polarization detection). This finding
suggests that the hot corona emitting the bulk of the detected X-rays is
elongated, rather than spherical. The X-ray polarization angle is consistent
with that found in sub-mm wavelengths. Since the sub-mm polarization was found
to be aligned with the jet direction in other X-ray binaries, this indicates
that the corona is elongated orthogonal to the jet.

Accretion (14) — X-ray astronomy (1810) — Low-mass x-ray binary stars (939) —
Polarimetry (1278) — Astrophysical black holes (98)

††facilities: IXPE, MAXI††software: ixpeobssim (Baldini et al., 2022), xspec
(Arnaud, 1996)

##  1 Introduction

Mass accretion is a fundamental process of energy extraction that powers some
of the brightest X-ray sources. It operates very efficiently for compact
objects, neutron stars and black holes (BHs), which accrete matter from a
nearby companion star. BH X-ray binaries display two major spectral states,
hard and soft, which are thought to be linked to different accretion regimes
(Zdziarski & Gierliński, 2004; Remillard & McClintock, 2006; Done et al.,
2007). In the soft state, the X-ray spectrum is dominated by a blackbody-like
emission that is believed to be produced by a geometrically thin, optically
thick accretion disk (Shakura & Sunyaev, 1973; Novikov & Thorne, 1973; Page &
Thorne, 1974). In the hard state, the spectrum constitutes a power-law-like
continuum, that is generally attributed to Comptonization in a medium with hot
electrons (a corona). At the transitions between the canonical states, the
hard-intermediate and soft-intermediate, as well as very high (or steep power-
law) states have been identified (Homan & Belloni, 2005; Belloni, 2010).

Transitions of BH X-ray binaries between various spectral states follow a
well-known pattern, however, our understanding of the accretion geometry and
physical mechanisms producing broadband emission in these states is still
incomplete. The geometry and location of the hot medium in the hard spectral
state is still a matter of debate (Poutanen et al., 2018; Bambi et al., 2021).
The structure of the accretion disk is unclear, its stability in the soft
state are puzzling (Dexter & Quataert, 2012; Jiang et al., 2013). Furthermore,
the conditions for the source to show the very high state are unknown (Done et
al., 2007). X-ray polarimetry is a new diagnostic that may help resolve
questions regarding the geometry of the emission region left unanswered by the
conventional tools of spectroscopy and timing.

Figure 1: Evolution of Swift J1727.8$–$1613 during the outburst. (a) MAXI
light curve in the 2–20 keV range. The time of the IXPE observation is marked
by the cyan vertical line. (b) Hardness ratio, i.e. the ratio of the photon
flux in the 4–10 keV band to that in the 2–4 keV band. (c) Hardness-flux
diagram. Blue crosses show the evolution of Swift J1727.8$–$1613 during the
current outburst. The position of the source on the diagram during IXPE
observation is marked with a black star. For comparison, we also plot the time
evolution of MAXI J1820+070 during its 2018 outburst with red crosses and GX
339$-$4 during its 2010 outburst with the orange crosses. The diagram (c)
shows that Swift J1727.8$–$1613 was observed in the hard state soon after the
turning point.

The Imaging X-ray Polarimetry Explorer (Weisskopf et al., 2022, IXPE) provided
the first significant detection of 2–8 keV X-ray polarization in the
archetypical BH Cyg X-1 (Krawczynski et al., 2022) – the source that
originally gave rise to the spectral classifications (Tananbaum et al., 1972).
The hot Comptonizing medium visible in the hard spectral state of the binary
appeared to be elongated in the plane of the accretion disk. The polarization
angle (PA) of X-ray emission was found to be consistent with the position
angle of the extended radio source (Miller-Jones et al., 2021), thus
confirming, for the first time, that the radio jets are launched along the
axis of the (inner) accretion disk. The polarization degree (PD) of $4.0\%\pm
0.2\%$ was higher than expected for the orbital inclination of the source
$i\approx 30\arcdeg$ (Miller-Jones et al., 2021). This can be explained by the
inner disk being more highly inclined than the outer disk (Krawczynski et al.,
2022) or by an outflow of the hot medium (Poutanen et al., 2023).

The BH X-ray binary Cyg X-3 became the second hard-state target of IXPE.
Similarly to Cyg X-1, the source swings between different spectral states,
which might be attributed to the same configurations of accreting matter as in
Cyg X-1. Contrary to early expectations, a high PD$=20.6\%\pm 0.3\%$ was
detected in the hard state (Veledina et al., 2023). The X-ray PA was found to
be shifted by $90\arcdeg$ with respect to the position angle of the discrete
radio ejections, as well as to the sub-mm polarization direction. These unique
polarization properties can be explained in terms of a pure reflection
scenario. The X-ray polarization observations revealed the presence of an
obscuring and reflecting envelope at high elevation above the disk plane,
suggesting this well-studied binary is a Galactic ultraluminous X-ray source.

The aforementioned hard-state sources belong to the class of high-mass X-ray
binaries, where the compact object captures matter from the wind of their
massive companion. The innermost accretion geometry of low-mass X-ray binary
BHs, where the disk forms via Roche lobe overflow, was not yet studied
polarimetrically. In this paper, we report on the first X-ray polarization
measurement of the Galactic X-ray binary Swift J1727.8$–$1613.

##  2 Discovery and Outburst

Swift J1727.8$–$1613 underwent a bright outburst that was detected on 2023 Aug
24 (Kennea & Swift Team, 2023; Negoro et al., 2023a). MAXI (Matsuoka et al.,
2009) monitoring of the source showed it reaching $\sim$7 Crab in the 2–20 keV
range (see Fig. 1a), triggering interest and rapid initiation of follow-up
multiwavelength campaigns by a number of ground-based and space observatories
(Negoro et al., 2023b; O’Connor et al., 2023; Wang & Bellm, 2023; Miller-Jones
et al., 2023; Baglio et al., 2023; Williams-Baldwin et al., 2023). The
beginning of the outburst was observed in the optical wavelengths about 4 days
earlier than the first X-ray trigger (Wang & Bellm, 2023). Optical
spectroscopy taken at the early outburst stages suggest the source is a low-
mass X-ray binary (Castro-Tirado et al., 2023), with signatures of an outflow
(Mata Sanchez & Muñoz-Darias, 2023). On the next and the third day after the
X-ray trigger, the X-ray spectral shape was identified to be consistent with
the typical hard-state power law with photon index $\Gamma\sim 1.3-1.7$, as
measured by the LEIA imager (Liu et al., 2023) and by the Mikhail Pavlinsky
ART-XC telescope (Sunyaev et al., 2023), respectively. Recent NICER
observations (taken 25 days after the beginning of the outburst) report
substantial contribution of the blackbody emission (of temperature $kT_{\rm
bb}=0.6$ keV) and softening of the spectrum ($\Gamma=2.4$, Bollemeijer et al.,
2023). X-ray light-curves revealed prominent quasi-periodic oscillations with
a slowly-increasing (on timescales of days) central frequency (Palmer &
Parsotan, 2023; Draghis et al., 2023; Bollemeijer et al., 2023).

The onset of the radio source detected at various frequencies shortly after
the X-ray trigger identified the presence of a jet (Miller-Jones et al., 2023;
Bright et al., 2023), with a flat spectrum (Vrtilek et al., 2023, Trushkin et
al., in prep.) typical to the hard-state sources. A subsequent sub-mm
detection and polarization measurements indicate its direction is nearly along
the North-South celestial axis (PA=$-4\fdg 1\pm 3\fdg 5$ on the date closest
to IXPE pointing; Vrtilek et al., 2023). The intrinsic optical polarization
also seems to be roughly aligned with the North-South direction (Kravtsov et
al., 2023). While the mass of the compact object has not been reliably
measured yet, all indirect signatures indicate the binary hosts a BH, making
it an intriguing target that can be used to probe the accretion geometry.

Fig. 1a shows the initial stages of the countrate evolution in Swift
J1727.8$–$1613 as seen by MAXI monitor,111http://maxi.riken.jp/ and the
typical fast rise profile can be clearly identified. The starting point for
the date corresponds to the initial X-ray trigger (which we will refer to as
the beginning of the X-ray outburst hereafter). A few days after the beginning
of the outburst, the source started to soften (see Fig. 1b). The source
evolution follows the well-known q-track pattern (Homan & Belloni, 2005) in
the hardness-flux diagram (blue symbols in Fig. 1c), albeit becoming much
brighter than the prominent low-mass X-ray binary MAXI J1820+070 (shown in
red) and the prototypical source GX 339–4 (orange). IXPE observed Swift
J1727.8$–$1613 at high, close to current maxima, fluxes, about 15 days after
the beginning of the outburst. According to the position in hardness-flux
diagram, the source started transition to the soft state and resided in the
hard-intermediate state (which we refer to as the hard state hereafter, Fig.
1c).

##  3 Observations and Data Reduction

IXPE is the first satellite mission dedicated to X-ray polarimetry in the 2–8
keV band (Weisskopf et al., 2022). It carries three X-ray telescopes, each
made of a Mirror Module Assembly (Ramsey et al., 2022) and a polarization-
sensitive gas-pixel detector unit (Soffitta et al., 2021; Baldini et al.,
2021), that enable imaging X-ray polarimetry of extended sources and a huge
increase of sensitivity for point-like sources. IXPE provides an angular
resolution of $\sim 30\arcsec$ (half-power diameter). The overlap of the
fields of view of the three detector units is circular with a diameter of 9′;
spectral resolution is better than 20% at 6 keV.

Figure 2: IXPE X-ray spectra of Swift J1727.8$–$1613\. Stokes $I$, $Q$, and
$U$ spectra are shown in the left, middle, and right panels, respectively.
Data from the three IXPE detectors are shown as crosses: det1 (blue), det2
(red), and det3 (green). The best-fit model polconst*powerlaw is shown with
corresponding lines. The lower sub-panels show the fit residuals.

IXPE observed Swift J1727.8$–$1613 from 2023 Sept 7 19:59:07 UTC to Sept 8
06:35:12 UTC for a livetime of $\sim$19 ks (Dovciak et al., 2023a, b). Level 2
data were downloaded from the IXPE archive at the
HEASARC222https://heasarc.gsfc.nasa.gov/docs/ixpe/archive/ and analyzed with
both ixpeobssim version 30.6.2 (Baldini et al., 2022) and HEASOFT/xspec
version 12.13.1d (Arnaud, 1996). IXPE observations of Swift J1727.8$–$1613
were carried out placing a “gray” filter in front of the detector (Ferrazzoli
et al., 2020; Soffitta et al., 2021). The filter was used to reduce (by a
factor of $\sim$10) the incident flux from a very bright source, especially at
low energies, to a level compatible with the dead-time of the detector units
and with the need to transmit all data within the allocated telemetry. Now
different response matrices have to be used for data analysis, which are
available both in the ixpeobssim package and in the HEASARC CALDB. Comparing
the polarization obtained by the pcube algorithm in ixpeobssim and the one
from xspec, we found that the latter provides more consistent results as it
fully takes into account the energy response of the instrument. This is
particularly important to correctly describe the low-energy response of the
IXPE detectors when the gray filter is used, as its transmission drops steeply
at low energies. In the following, we then present the results obtained with
xspec. Source events were extracted using a circle with 80″ radius centered on
the source. No background subtraction is necessary for sources as bright as
Swift J1727.8$–$1613, since in this case the IXPE background is dominated by
scattered source emission, even at large offset (hence, the background itself
is negligible and can be ignored, Di Marco et al., 2023).

Table 1: Energy dependence of polarization properties.

| Energy range (keV)
---|---
Parameter | 2–3 | 3–4 | 4–5 | 5–6 | 6–7 | 7–8 | 2–8
$Q/I$ (%) | $3.4\pm 0.5$ | $4.2\pm 0.3$ | $4.2\pm 0.4$ | $4.5\pm 0.5$ | $5.1\pm 0.8$ | $4.8\pm 1.3$ | $4.1\pm 0.2$
$U/I$ (%) | $0.9\pm 0.5$ | $0.3\pm 0.3$ | $0.1\pm 0.4$ | $0.0\pm 0.5$ | $-0.7\pm 0.8$ | $0.4\pm 1.3$ | $0.3\pm 0.2$
PD (%) | $3.5\pm 0.5$ | $4.2\pm 0.3$ | $4.2\pm 0.4$ | $4.5\pm 0.5$ | $5.1\pm 0.8$ | $4.8\pm 1.3$ | $4.1\pm 0.2$
PA (deg) | $7.4\pm 3.7$ | $2.2\pm 2.3$ | $0.7\pm 2.7$ | $-0.2\pm 3.4$ | $-3.7\pm 4.2$ | $2.3\pm 8.0$ | $2.2\pm 1.3$

Note. — Polarization characteristics are obtained from the $I$, $Q$, and $U$
Stokes parameters computed with PHA1, PHA1Q, and PHA1U algorithms in xpbin and
fitted with polconst*powerlaw model in given energy intervals. The
uncertainties are given at the 68.3% (1$\sigma$) confidence level for one
interesting parameter.

##  4 Polarization Results

We fitted Stokes $I$, $Q$, and $U$ spectra with xspec, using only simple
models because of the limited spectral capability of IXPE. We first tested a
polconst*diskbb model, representing an accretion disk consisting of multiple
blackbody components with a constant polarization, using the gain fit command
in xspec to fit the energy scale. We found an unacceptable fit,
$\chi^{2}=3509$ for 1329 d.o.f., driven by large residuals in the $I$
spectrum. Models for absorption remain unconstrained due to the relatively
high IXPE threshold (2 keV) and the use of the gray filter, which strongly
reduces the flux from the source at low energies. Then, we used a power-law
model assuming constant polarization polconst*powerlaw, using gain fit
command. The data with the best-fit model are shown in Fig. 2. The model gives
$\chi^{2}=1282$ for 1329 d.o.f. The best-fit power-law index is
$\Gamma=1.80^{+0.02}_{-0.01}$, with the polarization being PD=$4.1\%\pm 0.2\%$
at PA=$2\fdg 2\pm 1\fdg 3$. The errors are at 68% confidence level and
calculated assuming one interesting parameter with the steppar command of
xspec. The polarization from the source is detected at a $\sim 20\sigma$
confidence level.

Figure 3: Contour plots of the PD and PA in three energy bands measured with
xspec at 90% confidence level.

We then froze the spectral model to the one found for the 2–8 keV energy band
and fitted in the different energy ranges leaving only the polarization model
(either polconst or stokesconst) free to vary. The results are given in Table
1. The PD shows an increasing trend with energy, growing from about 3% in the
2–3 keV band to $\sim$5% above 5 keV. The PA does not show any significant
variations with energy. Contours plots of the PD and PA in three energy bands
are shown in Fig. 3.

We then checked for a possible energy dependence of the polarization by
substituting the polconst model with pollin, which assumes a linear dependence
of PD and PA on photon energy. However, we assumed an energy-independent PA
($\psi_{1}$ in xspec; fixing $\psi_{\rm slope}=0$), and allowed the PD to vary
with photon energy $E$ (keV) as $\mbox{PD}(E)=A_{1}+A_{\rm slope}(E-1)$. We
obtained a marginally better fit $\chi^{2}/\mathrm{d.o.f.}=1277/1328$ with the
F-test giving a probability of $3\%$ for a chance improvement. The best-fit
parameters are $A_{1}=3.0\%\pm 0.5\%$ and $A_{\rm slope}=0.34\pm 0.13$% keV-1,
and PA=$\psi_{1}=2\fdg 0\pm 1\fdg 3$.

##  5 Discussion

The exceptional brightness of the newly-discovered source Swift J1727.8$–$1613
is striking and can be compared to only a few historical BH transients: the
low- or intermediate-mass X-ray binaries A0620$-$00 (Elvis et al., 1975), V404
Cyg (Życki et al., 1999), V4641 Sgr (Revnivtsev et al., 2002), MAXI J1820+070
(Shidatsu et al., 2018), and 4U 1543$-$47 (Sánchez-Sierras et al., 2023). The
high X-ray fluxes of A0620$-$00 and MAXI J1820+070 are caused by the proximity
of these sources to the observer, and the inferred accretion rates fall in the
sub-Eddington category. In these cases, we see the innermost parts of the
disk/corona during the hard state. On the other hand, the luminosities of V404
Cyg, V4641 Sgr, and 4U 1543$-$47 are believed to reach near- and super-
Eddington values, and signatures of an optically thick outflow, that can cover
the innermost regions, have been detected (Revnivtsev et al., 2002; Motta et
al., 2017; Prabhakar et al., 2023). Analogs to these classes of sources can be
found in active galactic nuclei, whose geometry studies are now also enabled
by IXPE data. For the hard-spectrum sources where the innermost parts of the
accretion disk are visible, the X-ray polarization is found to be in a range
of about a few per cent (Cyg X-1, Krawczynski et al. 2022; NGC 4151, Gianolli
et al. 2023; MCG-05-23-16, Marinucci et al. 2022; Tagliacozzo et al. 2023; IC
4329A, Ingram et al. 2023) and aligned with the jet direction (wherever it is
constrained). For the sources where the central engine is obscured, the
observed spectrum is dominated by a reflection continuum, and high (PD$>20$%)
polarization orthogonal to the jet direction (polarization aligned with the
obscuring torus) has been found (Cyg X-3, Veledina et al. 2023; Circinus
galaxy, Ursini et al. 2023).

The relatively low X-ray PD found in Swift J1727.8$–$1613 (as compared to some
other accreting BHs) suggests the innermost parts of the accretion disk are
visible, and the geometry is similar to that of Cyg X-1 and Seyfert 1
galaxies. This is supported by the hardness-flux diagram (Fig. 1c) which
indicates that IXPE observed the source at the beginning of the hard to soft
state transition. By comparing the flux of Swift J1727.8$–$1613 at the turning
point (at which a source begins softening) with those of other BH X-ray
binaries MAXI J1820+070 situated at 3 kpc (Atri et al., 2020) and GX 339$-$4
at an estimated distance of $\sim$10 kpc (Zdziarski et al., 2019), we can
estimate that Swift J1727.8$–$1613 is closer than MAXI J1820+070 by a factor
of two (i.e. at a distance of about 1.5 kpc).

Relativistic ejections have not yet been observed in the source, hence the
direction of the jet is not known from radio images. The sub-mm polarization
signal however gives a clue as to the jet axis (oriented close to the North-
South direction, Vrtilek et al., 2023). It has been found that the intrinsic
polarization in the optical, sub-mm, and radio (after correction for Faraday
rotation) is aligned with the jet direction in the X-ray binaries during the
outburst (Cyg X-3, McCollough et al. in prep.; Lange et al., in prep.;
Veledina et al. 2023; XTE J1908+094, Curran et al. 2015; XTE J1550$-$564,
Migliori et al. 2017; MAXI J1820+070, Veledina et al. 2019; and V404 Cyg,
Kosenkov et al. 2017). If the jet direction in Swift J1727.8$–$1613 also
aligns with the detected optical and sub-mm polarization, then the X-ray
polarization is also aligned with the jet direction, supporting the analogy to
sub-Eddington, rather than super-Eddington sources.

The similarity of the X-ray PA to the sub-mm PA and, by extension, to the
position angle of the radio jet is related to the alignment of the jet
direction with the BH spin and the jet collimation mechanisms (McKinney et
al., 2013; Davis & Tchekhovskoy, 2020). If the BH spin is misaligned with the
binary orbital axis, the accretion disk/flow becomes warped and/or can
experience Lense-Thirring precession (Bardeen & Petterson, 1975; Stella &
Vietri, 1998; Fragile et al., 2007). Precession of the accretion flow may then
be visible in multiwavelength light-curves as prominent quasi-periodic
oscillations (Ingram et al., 2009; Veledina et al., 2013), which are indeed
detected in the X-ray timing data on Swift J1727.8$–$1613 (Palmer & Parsotan,
2023; Draghis et al., 2023). We clearly detect two harmonics of these
oscillations, at $1.340\pm 0.004$ and $2.74\pm 0.04$ Hz, in the IXPE flux data
(Ewing et al., in prep.). In the case of a substantial misalignment between
the orbital and BH spin axes and rapid Lense-Thirring precession of the inner
flow, the average (over the precession period) flow axis is aligned with the
spin axis. As a consequence, the average X-ray PA is parallel to the BH spin
axis. The outer parts of the disk are, on the other hand, expected to be
aligned with the orbital plane (e.g., as found in MAXI J1820+070, Poutanen et
al., 2022). The alignment of the jet direction with the BH spin in Swift
J1727.8$–$1613 also suggests that the jets are launched from the radii that
are either aligned with the BH spin axis or experience rapid precession around
it.

The similarity of the detected X-ray PD in Swift J1727.8$–$1613 and Cyg X-1
may be interpreted as similarity of inclinations of the inner parts of the
accretion disk in these systems, that translates to an intermediate
inclination of the source $i\sim 30\arcdeg-60\arcdeg$. This is further
supported by the absence of the X-ray dips. The inclination of the binary
orbit can be reliably measured only after the source reaches quiescence
(Casares & Jonker, 2014), and may help to distinguish between various
configurations of the accretion disk and corona. However, basic constraints on
the geometry can be made using these first X-ray polarization measurements.
While the X-ray PA and its relation to the sub-mm PA suggest that the X-ray
corona is extended along the disk (and not along the jet), the high
PD$\sim$4%, with some indication for increasing PDs with energy, argue against
spherical and lamppost coronal geometries (Krawczynski et al., 2022).

Swift J1727.8$–$1613 continues to gradually soften, in line with the q-pattern
in the hardness-flux diagram (Fig. 1c). Subsequent observations by IXPE
detecting the source in the soft and very high states may enable comparison to
the other BH X-ray binaries observed in polarized X-ray light (Ratheesh et
al., 2023; Rodriguez Cavero et al., 2023; Podgorny et al., 2023; Svoboda et
al., 2023; Dovciak et al., 2023c), providing independent constraints on the
inclination and calibrating the models of accretion disk structure and
radiative processes.

##  6 Summary

We obtained the first X-ray polarization measurement for the bright BH binary
Swift J1727.8$–$1613\. We find the time- and energy-averaged PD$=4.1\%\pm
0.2\%$ and PA$=2\fdg 2\pm 1\fdg 3$ (68% confidence) from xspec spectro-
polarimetric fits. There is no statistically significant dependence of the PA
on energy. A model that assumes a linear dependence of PD on energy gives a
slight improvement in $\chi^{2}$ significant at 97% confidence level and
results in a PD increasing with energy by 0.34% per keV.

The evolution of the source in the hardness-flux diagram and the observed
spectral slope indicate that the source was in the hard state during IXPE
observations, accreting at sub-Eddington rates, so its high X-ray brightness
is caused by the source proximity to the Earth. We estimate the distance to
the source to be about 1.5 kpc.

The alignment of X-ray, optical, and sub-mm polarization directions indicates
that the hot medium producing the X-ray continuum emission is extended in the
accretion disk plane (orthogonal to the jet). Our findings support the
expectation that the jets are launched orthogonal to the inner parts of the
accretion flow. By extension, this means that the direction of the inner parts
of the jet in Swift J1727.8$–$1613 are aligned with the BH spin.

The PD value is comparable to that found in the high-mass BH X-ray binary Cyg
X-1, and can, by similarity, constrain the inclination of the innermost parts
to be moderate, $i\sim$30°–60°. Also, the trend of PD with energy is
inconsistent with the geometry of the spherical or lamppost corona, even if it
is outflowing.

## Acknowledgments

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), and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto
Nazionale di Fisica Nucleare (INFN) in Italy. This research has made use of
the MAXI data provided by RIKEN, JAXA and the MAXI team.

A.V. thanks the Academy of Finland grant 355672 for support. M.D., J.S.,
J.Pod., and V.Kar. thank GACR project 21-06825X for the support and
institutional support from RVO:67985815. A.I. acknowledges support from the
Royal Society. H.K. acknowledges support by NASA grants 80NSSC22K1291,
80NSSC23K1041, and 80NSSC20K0329. The French contribution is supported by the
French Space Agency (Centre National d’Etude Spatiale, CNES) and by the High
Energy National Programme (PNHE) of the Centre National de la Recherche
Scientifique (CNRS). I.L. was supported by the NASA Postdoctoral Program at
the Marshall Space Flight Center, administered by Oak Ridge Associated
Universities under contract with NASA.

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