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Published June 21, 2024 | Version in press
Journal Article Open

Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE

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  • 1. ROR icon California Institute of Technology

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.

Copyright and License

© The Author(s), under exclusive licence to Springer Nature Limited 2024.

Acknowledgement

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).

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.

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.

Extended Data Fig. 1 Variation with time of flux and polarization for the IXPE Main observation.

Extended Data Fig. 2 X-ray light curves of Cyg X-3.

Extended Data Fig. 3 Radio and sub-mm light curves of Cyg X-3.

Extended Data Fig. 4 Radio-X-ray evolution track from historical radio and X-ray observations.

Extended Data Fig. 5 Broadband spectral energy distribution of Cyg X-3.

Extended Data Fig. 6 X-ray SED of Cyg X-3 from NICER.

Extended Data Fig. 7 Orbital phase dependence of polarization.

Extended Data Fig. 8 Modelling orbital variations of the PD and PA.

Extended Data Fig. 9 Detailed geometry of the reflecting funnel, its polarimetric characteristics, reflection and amplification factors.

Extended Data Fig. 10 Results of Monte-Carlo simulations.

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.

Conflict of Interest

The authors declare no competing interests.

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Additional details

Funding

National Aeronautics and Space Administration
NNM15AA18C
Agenzia Spaziale Italiana
ASI-OHBI-2017-12-I.0
Agenzia Spaziale Italiana
ASI-INAF-2017-12-H0
Agenzia Spaziale Italiana
ASI-INFN-2017.13-H0
National Institute for Astrophysics
Istituto Nazionale di Fisica Nucleare
European Research Council
307215
Smithsonian Astrophysical Observatory
01hfpy566
European Resuscitation Council
101004719
Agenzia Spaziale Italiana
I/028/12/7-2022
Ministero degli Affari Esteri
CN24GR08
Research Council of Finland
333112
Research Council of Finland
347003
Research Council of Finland
349144
Research Council of Finland
349373
Research Council of Finland
349906
Research Council of Finland
355672
UK Research and Innovation
Stephen Hawking Fellowship
National Aeronautics and Space Administration
80NSSC18K0264
National Aeronautics and Space Administration
027ka1x80
National Aeronautics and Space Administration
80NSSC21K1817
National Aeronautics and Space Administration
NNX16AC42G
German Academic Exchange Service
57525212
Royal Society
Czech Science Foundation
21-06825X
Czech Academy of Sciences
RVO:67985815
Department of Atomic Energy
12-R&D-TFR-5.02-0700
National Aeronautics and Space Administration
NAS8-03060
The Ministry of Education and Science of the Russian Federation
075-15-2022-26
The Ministry of Education and Science of the Russian Federation
13.MNPMU.21.0003
National Science Center
2019/35/B/ST9/03944

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