Abstract
The accretion of matter by compact objects can be inhibited by radiation pressure if the luminosity exceeds a critical value known as the Eddington limit. The discovery of ultraluminous X-ray sources has shown that accretion can proceed even when the apparent luminosity considerably exceeds this limit. A high apparent luminosity might be produced due to the geometric beaming of radiation by an outflow. The outflow half-opening angle, which determines the amplification due to beaming, has never been robustly constrained. Using the Imaging X-ray Polarimetry Explorer, we measured the X-ray polarization in the Galactic X-ray binary Cygnus X-3 (Cyg X-3). We found high, >20%, nearly energy-independent linear polarization orthogonal to the direction of the radio ejections. These properties unambiguously indicate the presence of a collimating outflow from the X-ray binary Cyg X-3 and constrain its half-opening angle to ≲15°. Thus, the source can be used as a laboratory for studying the supercritical accretion regime. This finding underscores the importance of X-ray polarimetry in advancing our understanding of accreting sources.
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Data availability
The IXPE, NuSTAR, NICER, INTEGRAL and Fermi data are freely available in the HEASARC Data Archive (https://heasarc.gsfc.nasa.gov). The SRG ART-XC data are available via ftp://hea.iki.rssi.ru/public/SRG/ART-XC/data/Cyg_X-3/artxc_cygx3_04-20keV_lcurve.qdp. The multiwavelength raw data are available on request from the individual observatories.
Code availability
The analysis and simulation software IXPEOBSSIM developed by the IXPE Collaboration and its documentation is available publicly through the web-page https://ixpeobssim.readthedocs.io/en/latest/?badge=latest.494. XSPEC is distributed and maintained under the aegis of the HEASARC and can be downloaded as part of HEAsoft from http://heasarc.gsfc.nasa.gov/docs/software/lheasoft/download.html. The MIR software package for the SMA data is available at https://lweb.cfa.harvard.edu/~cqi/mircook.html. Models of the polarized emission from the funnel are available via Zenodo at https://zenodo.org/records/10889892 (ref. 106). The STOKES code v.2.07 is available upon reasonable request from the authors.
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Acknowledgements
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, with industry partner Ball Aerospace (Contract NNM15AA18C). The Italian contribution is supported by ASI (Contract ASI-OHBI-2017-12-I.0 and Agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0) and its Space Science Data Center and by the Italian National Institute for Astrophysics and the Italian National Institute for Nuclear Physics. For the AMI observations, we thank the staff of the Mullard Radio Astronomy Observatory, University of Cambridge, for their support in the maintenance and operation of the telescope, and we acknowledge support from the European Research Council (Grant No. ERC-2012-StG-307215 LODESTONE). The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. SMA is on Maunakea, which is a culturally important site for the indigenous Hawaiian people; we are privileged to study the cosmos from its summit. This work is partly based on observations with the 100-m telescope of the Max Planck Institute for Radio Astronomy at Effelsberg. The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 101004719; ORP). The AGILE Mission is funded by ASI with scientific and programmatic participation by the Italian National Institute for Astrophysics and the Italian National Institute for Nuclear Physics. This investigation was supported by the ASI (Grant No. I/028/12/7-2022). We thank H. Feng for providing the data on the representative ULX models. F. Muleri, A.D.M., F.L.M., E. Costa, P. Soffitta, S.F. and R.F. are partially supported by the Italian Ministry of Foreign Affairs (Grant No. CN24GR08, GRBAXP: Guangxi-Rome Bilateral Agreement for X-ray Polarimetry in Astrophysics). A.V., J. Poutanen and S.S.T. acknowledge support from the Academy of Finland (Grant Nos. 333112, 347003, 349144, 349373, 349906 and 355672). A.A.M. is supported by the Stephen Hawking fellowship from UK Research and Innovation. H.K. and N.R.C. acknowledge NASA support (Grant Nos. 80NSSC18K0264, 80NSSC22K1291, 80NSSC21K1817 and NNX16AC42G). V.D. thanks the German Academic Exchange Service (Travel Grant No. 57525212). A.I. acknowledges support from the Royal Society. J. Podgorný, M.D., J.S. and V.K. give thanks for support from the Czech Science Foundation (Project 21-06825X) and institutional support from the Astronomical Institute of the Czech Academy of Sciences (Project RVO:67985815). We thank the staff of the GMRT who made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. R.K. acknowledges the support of the Department of Atomic Energy, Government of India (Project No. 12-R&D-TFR-5.02-0700). M.M. is supported by NASA (Contract NAS8-03060). S.A.T. is supported by the Ministry of Science and Higher Education of the Russian Federation (Grant No. 075-15-2022-262; 13.MNPMU.21.0003). A.A.Z. acknowledges support from the Polish National Science Center (Grant No. 2019/35/B/ST9/03944).
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Authors and Affiliations
Contributions
A.V. led the modelling of the data and the writing of the paper. F. Muleri led the analysis of the IXPE data. J. Poutanen led the analytical modelling and contributed to the writing of the paper. J. Podgorný performed the Monte Carlo simulations in support of the modelling. M.D. led the work of the IXPE Topical Working Group on Accreting Stellar-mass Black Holes. A.D.R., E. Churazov, P.K. and R.A.S. contributed with parts of the paper and its content. F.C., A.D.M., S.V.F., H.K., F.L.M., A.A.L., S.V.M., A.R., N.R.C., J.F.S., S.S.T., A.A.Z. and J.J.E.K., I.A.M., G.P. and C.P. contributed to planning, reducing and analysing the X-ray and γ-ray data. V.L., A.A.M. and D.M. contributed to analytical estimates and modelling. J.S.B., N. Bursov, E.E., D.A.G., M.G., R.K., A.K., M.M., N.N., M. Pilia, R.R., S.R., A.S., J.S., S.A.T. and P.T. contributed with radio and submillimetre data. S.B., E. Costa, J.A.G., A.I., F. Marin, G.M., P. Soffitta, F. Tombesi, F.U., M.C.W. and K.W. contributed with discussions of the methods and conclusions. The remaining authors contributed to the design, science case of the IXPE mission and the planning of observations relevant to the present paper.
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Extended data
Extended Data Fig. 1 Variation with time of flux and polarization for the IXPE Main observation.
(A) The total rate in the 2–8 keV energy range, binned in time intervals of 500 s. The PD (B) and the PA (C) are averaged over one orbit, as defined by the ephemeris of21. Dashed horizontal lines are the average values. (D) The hardness ratio defined as the ratio of the difference in the IXPE count rates in the 4–8 and 2–4 keV energy bands to their sum in 1000 s time bins. Alternating vertical bands identify different orbits. Data are presented as mean values over the time bin and the error bars correspond to 1σ confidence levels.
Extended Data Fig. 2 X-ray light curves of Cyg X-3.
X-ray count rates normalised to the average during the Main observation obtained by three X-ray telescopes: NuSTAR (A), SRG/ART-XC (B) and INTEGRAL/ISGRI (C). The IXPE exposure covers the entire duration of the displayed observations.
Extended Data Fig. 3 Radio and sub-mm light curves of Cyg X-3.
The light curves of the source around the dates of Main (panels A-C) and ToO (panels D and E) observations, as obtained with various telescopes. IXPE dates are marked with blue stripes. Note high intraday variations of the radio flux caused by the orbital variability. Data are given as the mean values with error bars corresponding to their variance.
Extended Data Fig. 4 Radio-X-ray evolution track from historical radio and X-ray observations.
Grey points constitute data analysed in107. Spectral states are indicated with red. Blue and orange stars indicate the fluxes during the Main and ToO observations, respectively.
Extended Data Fig. 5 Broadband spectral energy distribution of Cyg X-3.
The SED for the Main (blue) and ToO (orange) observations are from the facilities described in the text. Error bars correspond to 1σ levels.
Extended Data Fig. 6 X-ray SED of Cyg X-3 from NICER.
Orbital phase-folded X-ray spectra are taken during the contemporaneous observations in the Main run. Spectra from different phase intervals are presented in different colours.
Extended Data Fig. 7 Orbital phase dependence of polarization.
The PD (A)–(C) and PA (D)–(F) in different energy bands (2–3.5 keV, A, D; 3.5–6 keV, B, E; 6–8 keV, C, F) for the Main (in blue) and ToO (in orange) observations are shown. Orbital profiles of IXPE flux are shown in each panel as shaded areas. Error bars correspond to 1σ uncertainty level.
Extended Data Fig. 8 Modelling orbital variations of the PD and PA.
(A) Geometry of the reflector. (B) Dependence of the PD and PA in the 3.5–6 keV band on orbital phase for the Main observation is shown with blue crosses. The red curve is the model of the reflection from a bow shock.
Extended Data Fig. 9 Detailed geometry of the reflecting funnel, its polarimetric characteristics, reflection and amplification factors.
(A) Geometry of the funnel is shown with L being the lowest visible point for the given inclination i, and the angle α* is its colatitude. (B) The contour plots of constant PD (in %) for the fixed observer inclination (i = 30∘), as function of the model parameters (α, R). The region above α = i is not allowed because the central source would be visible. The region below ρ = 1 curve (that is \(R=1/\sin \alpha\)) corresponds to an outflow converging towards the axis, which is not possible. Red contours show the allowed model parameters. (C) Dependence of the solid angle of the reflecting surface (Ωrefl/2π, red solid curve) and the factor determining the intrinsic luminosity (1 + ΩULX/Ωrefl, blue dashed curve) on the angle α.
Extended Data Fig. 10 Results of Monte-Carlo simulations.
(A) The geometry of the reflector (elliptical torus in blue) and main parameters of the funnel explored by the Monte-Carlo modelling. (B) The simulated 2–8 keV PD versus observer’s inclination and half-opening angle of the torus for b = ρ/4, τe = 7 and NHe = 8.5 × 1023cm−2 (the same display as in Fig. 4 for the analytical model). The black rectangles and white dashed lines mark the regions where the reprocessed component gives PD = 21 ± 3%.
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Veledina, A., Muleri, F., Poutanen, J. et al. Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE. Nat Astron 8, 1031–1046 (2024). https://doi.org/10.1038/s41550-024-02294-9
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DOI: https://doi.org/10.1038/s41550-024-02294-9
