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

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).

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.

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.