Discovery of X-Ray Polarization from the Black Hole Transient Swift J1727.8−1613
Alexandra Veledina1,2 , Fabio Muleri3 , Michal Dovčiak4 , Juri Poutanen1 , Ajay Ratheesh3 , Fiamma Capitanio3 ,
Giorgio Matt5 , Paolo Soffitta3 , Allyn F. Tennant6 , Michela Negro7 , Philip Kaaret6 , Enrico Costa3 , Adam Ingram8 ,
Jirí̌ Svoboda4 , Henric Krawczynski9 , Stefano Bianchi5 , James F. Steiner10 , Javier A. García11 , Vadim Kravtsov1 ,
Anagha P. Nitindala1 , Melissa Ewing8 , Guglielmo Mastroserio12 , Andrea Marinucci13 , Francesco Ursini5 ,
Francesco Tombesi14,15,16 , Sergey S. Tsygankov1 , Yi-Jung Yang17,18 , Martin C. Weisskopf6 , Sergei A. Trushkin19 ,
Elise Egron12 , Maria Noemi Iacolina20 , Maura Pilia12 , Lorenzo Marra5 , Romana Mikušincová5 , Edward Nathan21 ,
Maxime Parra5,22 , Pierre-Olivier Petrucci22 , Jakub Podgorný4,23,24 , Stefano Tugliani25,26 , Silvia Zane27 ,
Wenda Zhang28 , Iván Agudo29 , Lucio A. Antonelli30,31 , Matteo Bachetti12 , Luca Baldini32,33 ,
Wayne H. Baumgartner6 , Ronaldo Bellazzini32 , Stephen D. Bongiorno6 , Raffaella Bonino25,26 , Alessandro Brez32 ,
Niccolò Bucciantini34,35,36 , Simone Castellano32 , Elisabetta Cavazzuti13 , Chien-Ting Chen37 , Stefano Ciprini15,31 ,
Alessandra De Rosa3 , Ettore Del Monte3 , Laura Di Gesu13 , Niccolò Di Lalla38 , Alessandro Di Marco3 ,
Immacolata Donnarumma13 , Victor Doroshenko39 , Steven R. Ehlert6 , Teruaki Enoto40 , Yuri Evangelista3 ,
Sergio Fabiani3 , Riccardo Ferrazzoli3 , Shuichi Gunji41 , Kiyoshi Hayashida42,57, Jeremy Heyl43 , Wataru Iwakiri44 ,
Svetlana G. Jorstad45,46 , Vladimir Karas4 , Fabian Kislat47 , Takao Kitaguchi40, Jeffery J. Kolodziejczak6 ,
Fabio La Monaca3 , Luca Latronico25 , Ioannis Liodakis6 , Simone Maldera25 , Alberto Manfreda48 , Frédéric Marin23 ,
Alan P. Marscher45 , Herman L. Marshall49 , Francesco Massaro25,26 , Ikuyuki Mitsuishi50, Tsunefumi Mizuno51 ,
Chi-Yung Ng17 , Stephen L. O’Dell6 , Nicola Omodei38 , Chiara Oppedisano25 , Alessandro Papitto30 ,
George G. Pavlov52 , Abel L. Peirson38 , Matteo Perri30,31 , Melissa Pesce-Rollins32 , Andrea Possenti12 ,
Simonetta Puccetti31 , Brian D. Ramsey6 , John Rankin3 , Oliver J. Roberts37 , Roger W. Romani38 , Carmelo Sgrò32 ,
Patrick Slane10 , Gloria Spandre32 , Douglas A. Swartz37 , Toru Tamagawa40 , Fabrizio Tavecchio53 ,
Roberto Taverna54 , Yuzuru Tawara50, Nicholas E. Thomas7,15,16 , Alessio Trois12 , Roberto Turolla27,54 , Jacco Vink55 ,
Kinwah Wu27 , and Fei Xie3,56
1 Department of Physics and Astronomy, FI-20014 University of Turku, Finland
2 Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden
3 INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy
4 Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
5 Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy
6 NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
7 Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
8 School of Mathematics, Statistics, and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
9 Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
10 Center for Astrophysics, Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
11 X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
12 INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy
13 Agenzia Spaziale Italiana, Via del Politecnico snc, I-00133 Roma, Italy
14 Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata,” Via della Ricerca Scientifica 1, I-00133 Roma, Italy
15 Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata,” Via della Ricerca Scientifica 1, I-00133 Roma, Italy
16 Department of Astronomy, University of Maryland, College Park, MD 20742, USA
17 Department of Physics, The University of Hong Kong, Pokfulam Rd., Hong Kong
18 Laboratory for Space Research, The University of Hong Kong, Cyberport 4, Hong Kong
19 Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, 369167, Karachayevo-Cherkessia, Russia
20 Agenzia Spaziale Italiana, Via della Scienza 5, I-09047, Selargius (CA), Italy
21 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
22 Université Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France
23 Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
24 Astronomical Institute, Charles University, V Holešovičkách 2, CZ-18000, Prague, Czech Republic
25 Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, I-10125, Torino, Italy
26 Dipartimento di Fisica, Universitá degli Studi di Torino, Via Pietro Giuria 1, I-10125 Torino, Italy
27 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
28 National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Rd., Beijing 100101, Peopleʼs Republic of China
29 Instituto de Astrofísicade Andalucía—CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
30 INAF Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone (RM), Italy
31 Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, I-00133 Roma, Italy
32 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy
33 Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy
34 INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
35 Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via Sansone 1, I-50019 Sesto Fiorentino (FI), Italy
36 Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, I-50019 Sesto Fiorentino (FI), Italy
37 Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA
38 Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA
39 Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany
40 RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
41 Yamagata University,1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
42 Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
The Astrophysical Journal Letters, 958:L16 (8pp), 2023 November 20 https://doi.org/10.3847/2041-8213/ad0781
© 2023. The Author(s). Published by the American Astronomical Society.
1
43 University of British Columbia, Vancouver, BC V6T 1Z4, Canada
44 International Center for Hadron Astrophysics, Chiba University, Chiba 263-8522, Japan
45 Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
46 Department of Astrophysics, St. Petersburg State University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
47 Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
48 Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Strada Comunale Cinthia, I-80126 Napoli, Italy
49 MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
50 Graduate School of Science, Division of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
51 Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
52 Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16801, USA
53 INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
54 Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, I-35131 Padova, Italy
55 Anton Pannekoek Institute for Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
56 Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, Peopleʼs Republic of
China
Received 2023 September 26; revised 2023 October 24; accepted 2023 October 25; published 2023 November 21
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%± 0.2% and a polarization angle of 2°.2± 1°.3 (errors at 68% confidence level; this translates to ∼20σ
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
submillimeter wavelengths. Since the submillimeter 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.
Unified Astronomy Thesaurus concepts: Accretion (14); X-ray astronomy (1810); Low-mass x-ray binary stars
(939); Polarimetry (1278); Astrophysical black holes (98)
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 & McClin-
tock 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 (Novikov & Thorne 1973; Shakura & Sunyaev
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 under-
standing 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 is 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.
The Imaging X-ray Polarimetry Explorer (IXPE; Weisskopf
et al. 2022) provided the first significant detection of 2–8 keV
X-ray polarization in the archetypical BH Cyg X-1 (Krawc-
zynski 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%± 0.2% was higher than expected for the orbital
inclination of the source i≈ 30° (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%± 0.3% was
detected in the hard state (Veledina et al. 2023). The X-ray PA
was found to be shifted by 90° with respect to the position
angle of the discrete radio ejections, as well as to the
submillimeter polarization direction. These unique polarization
properties can be explained in terms of a pure reflection
scenario. The X-ray polarization observations revealed the
57 Deceased.
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The Astrophysical Journal Letters, 958:L16 (8pp), 2023 November 20 Veledina et al.
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 August 24 (Negoro et al. 2023a; Kennea &
Swift Team 2023). MAXI (Matsuoka et al. 2009) monitoring of
the source showed it reaching ∼7 Crab in the 2–20 keV range
(see Figure 1(a)), triggering interest and rapid initiation of
follow-up multiwavelength campaigns by a number of ground-
based and space observatories (Baglio et al. 2023; Miller-Jones
et al. 2023; Negoro et al. 2023b; O’Connor et al. 2023; Wang
& Bellm 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
Γ∼ 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 disk blackbody emission (of temperature
kTdiskbb= 0.6 keV) and softening of the spectrum (Γ= 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 (Bollemeijer et al.
2023; Draghis et al. 2023; Palmer & Parsotan 2023).
The onset of the radio source detected at various frequencies
shortly after the X-ray trigger identified the presence of a jet
(Bright et al. 2023; Miller-Jones et al. 2023), with a flat
spectrum (Vrtilek et al. 2023; S. Trushkin et al. 2023, in
preparation) typical to the hard-state sources. A subsequent
submillimeter detection and polarization measurements indi-
cate its direction is nearly along the north–south celestial axis
(PA=− 4°.1± 3°.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.
Figure 1(a) shows the initial stages of the countrate
evolution in Swift J1727.8−1613 as seen by MAXI
monitor,58 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 Figure 1(b)). The
source evolution follows the well-known q-track pattern
(Homan & Belloni 2005) in the hardness–flux diagram (blue
symbols in Figure 1(c)), 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; Figure 1(c)).
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.
58 http://maxi.riken.jp/
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The Astrophysical Journal Letters, 958:L16 (8pp), 2023 November 20 Veledina et al.
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 (Baldini et al. 2021; Soffitta et al. 2021),
which enable imaging X-ray polarimetry of extended sources
and a huge increase of sensitivity for point-like sources. IXPE
provides an angular resolution of ∼30″ (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.
IXPE observed Swift J1727.8−1613 from 2023 September 7
19:59:07 UTC to September 8 06:35:12 UTC for a live time of
∼19 ks (Dovciak et al. 2023a, 2023b). Level 2 data were
downloaded from the IXPE archive at the HEASARC59 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 ∼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).
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, χ2= 3509 for 1329 degrees of freedom (dof),
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 Figure 2. The model
gives χ2= 1282 for 1329 dof. The best-fit power-law index is
G = -+1.80 0.010.02, with the polarization being PD= 4.1%± 0.2%
at PA= 2°.2± 1°.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 ∼20σ 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 ∼5% above 5 keV. We note
that the contribution of the disk with the inner temperature
kTdiskbb∼ 0.5 keV (Bollemeijer et al. 2023; Draghis et al. 2023)
cannot be ruled out in the lower-energy bin and might cause the
reduction of PD here. At the same time, the PD at higher
energies, which we attribute to the corona, is rather constant.
The PA does not show any significant variations with energy.
Contours plots of the PD and PA in three energy bands are
shown in Figure 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 (ψ1 in XSPEC; fixing ψslope= 0), and allowed
the PD to vary with photon energy E (keV) as PD
(E)= A1+ Aslope(E− 1). We obtained a marginally better fit
χ2/dof=1277/1328 with the F-test giving a probability of 3%
for a chance improvement. The best-fit parameters are
A1= 3.0%± 0.5% and Aslope= 0.34± 0.13% keV
−1, and
PA=ψ1= 2°.0± 1°.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), GRS 1915+105
(Sazonov et al. 1994), 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 GRS 1915+105, 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; Miller et al.
2020; 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 percent (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) has59 https://heasarc.gsfc.nasa.gov/docs/ixpe/archive/
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The Astrophysical Journal Letters, 958:L16 (8pp), 2023 November 20 Veledina et al.
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 (Figure 1(c))
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 ∼10 kpc (Zdziarski et al.
2019), we can estimate that Swift J1727.8−1613 is closer than
MAXI J1820+070 by a factor of 2 (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 submillimeter 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, submillimeter, and radio
(after correction for Faraday rotation) is aligned with the jet
direction in the X-ray binaries during the outburst (Cyg X-3,
M. McCollough et al. 2023, in preparation; A. Lange et al.
2023, in preparation; 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 submillimeter
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 submillimeter 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 (Draghis et al. 2023; Palmer & Parsotan 2023). We
clearly detect two harmonics of these oscillations, at
1.340± 0.004 and 2.74± 0.04 Hz, in the IXPE flux data (M.
Ewing et al. 2023, in preparation). 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
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 subpanels show the fit residuals.
5
The Astrophysical Journal Letters, 958:L16 (8pp), 2023 November 20 Veledina et al.
systems, that translates to an intermediate inclination of the
source i∼ 30°–60°. 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 submillimeter PA suggest that
the X-ray corona is extended along the disk (and not along the
jet), the high PD∼ 4%, either constant or increasing with
energy, argue against spherical and lamppost coronal
geometries (Krawczynski et al. 2022; Zhang et al. 2022).
Swift J1727.8−1613 continues to gradually soften, in line
with the q-pattern in the hardness–flux diagram (Figure 1(c)).
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 (Dovciak
et al. 2023c; Marra et al. 2023; Podgorny et al. 2023; Ratheesh
et al. 2023; Rodriguez Cavero et al. 2023; Svoboda et al. 2023),
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%± 0.2% and PA= 2°.2± 1°.3
(68% confidence) from XSPEC spectropolarimetric 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 χ2 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 submillimeter
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∼ 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-2022-13-I.0, agreements ASI-INAF-2022-
19-HH.0 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).
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.
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 ± 0.5 4.2 ± 0.3 4.2 ± 0.4 4.5 ± 0.5 5.1 ± 0.8 4.8 ± 1.3 4.1 ± 0.2
U/I (%) 0.9 ± 0.5 0.3 ± 0.3 0.1 ± 0.4 0.0 ± 0.5 −0.7 ± 0.8 0.4 ± 1.3 0.3 ± 0.2
PD (%) 3.5 ± 0.5 4.2 ± 0.3 4.2 ± 0.4 4.5 ± 0.5 5.1 ± 0.8 4.8 ± 1.3 4.1 ± 0.2
PA (deg) 7.4 ± 3.7 2.2 ± 2.3 0.7 ± 2.7 −0.2 ± 3.4 −3.7 ± 4.2 2.3 ± 8.0 2.2 ± 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% (1σ) confidence level for one interesting parameter.
Figure 3. Contour plots of the PD and PA in three energy bands measured with
XSPEC at 90% confidence level.
6
The Astrophysical Journal Letters, 958:L16 (8pp), 2023 November 20 Veledina et al.
V.Kra. acknowledges support from the Finnish Cultural
Foundation. 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.
Facilities: IXPE, MAXI.
Software:IXPEOBSSIM (Baldini et al. 2022), XSPEC (Arnaud
1996).
ORCID iDs
Alexandra Veledina https://orcid.org/0000-0002-5767-7253
Fabio Muleri https://orcid.org/0000-0003-3331-3794
Michal Dovčiak https://orcid.org/0000-0003-0079-1239
Juri Poutanen https://orcid.org/0000-0002-0983-0049
Ajay Ratheesh https://orcid.org/0000-0003-0411-4243
Fiamma Capitanio https://orcid.org/0000-0002-6384-3027
Giorgio Matt https://orcid.org/0000-0002-2152-0916
Paolo Soffitta https://orcid.org/0000-0002-7781-4104
Allyn F. Tennant https://orcid.org/0000-0002-9443-6774
Michela Negro https://orcid.org/0000-0002-6548-5622
Philip Kaaret https://orcid.org/0000-0002-3638-0637
Enrico Costa https://orcid.org/0000-0003-4925-8523
Adam Ingram https://orcid.org/0000-0002-5311-9078
Jirí̌ Svoboda https://orcid.org/0000-0003-2931-0742
Henric Krawczynski https://orcid.org/0000-0002-
1084-6507
Stefano Bianchi https://orcid.org/0000-0002-4622-4240
James F. Steiner https://orcid.org/0000-0002-5872-6061
Javier A. García https://orcid.org/0000-0003-3828-2448
Vadim Kravtsov https://orcid.org/0000-0002-7502-3173
Anagha P. Nitindala https://orcid.org/0009-0002-
7109-0202
Melissa Ewing https://orcid.org/0000-0001-9349-8271
Guglielmo Mastroserio https://orcid.org/0000-0003-
4216-7936
Andrea Marinucci https://orcid.org/0000-0002-2055-4946
Francesco Ursini https://orcid.org/0000-0001-9442-7897
Francesco Tombesi https://orcid.org/0000-0002-6562-8654
Sergey S. Tsygankov https://orcid.org/0000-0002-
9679-0793
Yi-Jung Yang https://orcid.org/0000-0001-9108-573X
Martin C. Weisskopf https://orcid.org/0000-0002-
5270-4240
Sergei A. Trushkin https://orcid.org/0000-0002-7586-5856
Elise Egron https://orcid.org/0000-0002-1532-4142
Maria Noemi Iacolina https://orcid.org/0000-0003-
4564-3416
Maura Pilia https://orcid.org/0000-0001-7397-8091
Lorenzo Marra https://orcid.org/0009-0001-4644-194X
Romana Mikušincová https://orcid.org/0000-0001-
7374-843X
Edward Nathan https://orcid.org/0000-0002-9633-9193
Maxime Parra https://orcid.org/0009-0003-8610-853X
Pierre-Olivier Petrucci https://orcid.org/0000-0001-
6061-3480
Jakub Podgorný https://orcid.org/0000-0001-5418-291X
Stefano Tugliani https://orcid.org/0000-0002-3318-9036
Silvia Zane https://orcid.org/0000-0001-5326-880X
Wenda Zhang https://orcid.org/0000-0003-1702-4917
Iván Agudo https://orcid.org/0000-0002-3777-6182
Lucio A. Antonelli https://orcid.org/0000-0002-5037-9034
Matteo Bachetti https://orcid.org/0000-0002-4576-9337
Luca Baldini https://orcid.org/0000-0002-9785-7726
Wayne H. Baumgartner https://orcid.org/0000-0002-
5106-0463
Ronaldo Bellazzini https://orcid.org/0000-0002-2469-7063
Stephen D. Bongiorno https://orcid.org/0000-0002-0901-2097
Raffaella Bonino https://orcid.org/0000-0002-4264-1215
Alessandro Brez https://orcid.org/0000-0002-9460-1821
Niccolò Bucciantini https://orcid.org/0000-0002-8848-1392
Simone Castellano https://orcid.org/0000-0003-1111-4292
Elisabetta Cavazzuti https://orcid.org/0000-0001-7150-9638
Chien-Ting Chen https://orcid.org/0000-0002-4945-5079
Stefano Ciprini https://orcid.org/0000-0002-0712-2479
Alessandra De Rosa https://orcid.org/0000-0001-5668-6863
Ettore Del Monte https://orcid.org/0000-0002-3013-6334
Laura Di Gesu https://orcid.org/0000-0002-5614-5028
Niccolò Di Lalla https://orcid.org/0000-0002-7574-1298
Alessandro Di Marco https://orcid.org/0000-0003-0331-3259
Immacolata Donnarumma https://orcid.org/0000-0002-
4700-4549
Victor Doroshenko https://orcid.org/0000-0001-8162-1105
Steven R. Ehlert https://orcid.org/0000-0003-4420-2838
Teruaki Enoto https://orcid.org/0000-0003-1244-3100
Yuri Evangelista https://orcid.org/0000-0001-6096-6710
Sergio Fabiani https://orcid.org/0000-0003-1533-0283
Riccardo Ferrazzoli https://orcid.org/0000-0003-1074-8605
Shuichi Gunji https://orcid.org/0000-0002-5881-2445
Jeremy Heyl https://orcid.org/0000-0001-9739-367X
Wataru Iwakiri https://orcid.org/0000-0002-0207-9010
Svetlana G. Jorstad https://orcid.org/0000-0001-6158-1708
Vladimir Karas https://orcid.org/0000-0002-5760-0459
Fabian Kislat https://orcid.org/0000-0001-7477-0380
Jeffery J. Kolodziejczak https://orcid.org/0000-0002-
0110-6136
Fabio La Monaca https://orcid.org/0000-0001-8916-4156
Luca Latronico https://orcid.org/0000-0002-0984-1856
Ioannis Liodakis https://orcid.org/0000-0001-9200-4006
Simone Maldera https://orcid.org/0000-0002-0698-4421
Alberto Manfreda https://orcid.org/0000-0002-0998-4953
Frédéric Marin https://orcid.org/0000-0003-4952-0835
Alan P. Marscher https://orcid.org/0000-0001-7396-3332
Herman L. Marshall https://orcid.org/0000-0002-6492-1293
Francesco Massaro https://orcid.org/0000-0002-1704-9850
Tsunefumi Mizuno https://orcid.org/0000-0001-7263-0296
Chi-Yung Ng https://orcid.org/0000-0002-5847-2612
Stephen L. O’Dell https://orcid.org/0000-0002-1868-8056
Nicola Omodei https://orcid.org/0000-0002-5448-7577
Chiara Oppedisano https://orcid.org/0000-0001-6194-4601
Alessandro Papitto https://orcid.org/0000-0001-6289-7413
George G. Pavlov https://orcid.org/0000-0002-7481-5259
Abel L. Peirson https://orcid.org/0000-0001-6292-1911
Matteo Perri https://orcid.org/0000-0003-3613-4409
Melissa Pesce-Rollins https://orcid.org/0000-0003-1790-8018
Andrea Possenti https://orcid.org/0000-0001-5902-3731
Simonetta Puccetti https://orcid.org/0000-0002-2734-7835
Brian D. Ramsey https://orcid.org/0000-0003-1548-1524
John Rankin https://orcid.org/0000-0002-9774-0560
Oliver J. Roberts https://orcid.org/0000-0002-7150-9061
Roger W. Romani https://orcid.org/0000-0001-6711-3286
Carmelo Sgrò https://orcid.org/0000-0001-5676-6214
Patrick Slane https://orcid.org/0000-0002-6986-6756
7
The Astrophysical Journal Letters, 958:L16 (8pp), 2023 November 20 Veledina et al.
Gloria Spandre https://orcid.org/0000-0003-0802-3453
Douglas A. Swartz https://orcid.org/0000-0002-2954-4461
Toru Tamagawa https://orcid.org/0000-0002-8801-6263
Fabrizio Tavecchio https://orcid.org/0000-0003-0256-0995
Roberto Taverna https://orcid.org/0000-0002-1768-618X
Nicholas E. Thomas https://orcid.org/0000-0003-
0411-4606
Alessio Trois https://orcid.org/0000-0002-3180-6002
Roberto Turolla https://orcid.org/0000-0003-3977-8760
Jacco Vink https://orcid.org/0000-0002-4708-4219
Kinwah Wu https://orcid.org/0000-0002-7568-8765
Fei Xie https://orcid.org/0000-0002-0105-5826
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