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Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

Romana Mikušincová, Alexandra Veledina, Fabio Muleri, Raul Ciancarella, Andrzej Zdziarski, David A. Green, Michael McCollough, Henric Krawczynski, James F. Steiner, Michal Dovčiak, Varpu Ahlberg, Stefano Bianchi, Alessandro Di Marco, Javier A. García, Adam Ingram, Philip Kaaret, Timothy Kallman, Hu Kun, Fabio La Monaca, Alexander Lange, Vladislav Loktev, Guglielmo Mastroserio, Giorgio Matt, Razieh Emami, Pierre-Olivier Petrucci, Jakub Podgorný, Juri Poutanen, Ajay Ratheesh, Nicole Rodriguez, Jiří Svoboda, Francesco Tombesi, Francesco Ursini, Iván Agudo, Lucio A. Antonelli, Matteo Bachetti, Luca Baldini, Wayne H. Baumgartner, Ronaldo Bellazzini, Stephen D. Bongiorno, Raffaella Bonino, Alessandro Brez, Niccolò Bucciantini, Fiamma Capitanio, Simone Castellano, Elisabetta Cavazzuti, Chien-Ting Chen, Stefano Ciprini, Enrico Costa, Alessandra De Rosa, Ettore Del Monte, Laura Di Gesu, Niccolò Di Lalla, Immacolata Donnarumma, Victor Doroshenko, Steven R. Ehlert, Teruaki Enoto, Yuri Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Shuichi Gunji, Kiyoshi Hayashida, Jeremy Heyl, Wataru Iwakiri, Svetlana G. Jorstad, Vladimir Karas, Fabian Kislat, Takao Kitaguchi, Jeffery J. Kolodziejczak, Luca Latronico, Ioannis Liodakis, Simone Maldera, Alberto Manfreda, Frédéric Marin, Andrea Marinucci, Alan P. Marscher, Herman L. Marshall, Francesco Massaro, Ikuyuki Mitsuishi, Tsunefumi Mizuno, Michela Negro, Chi-Yung Ng, Stephen L. O'Dell, Nicola Omodei, Chiara Oppedisano, Alessandro Papitto, George G. Pavlov, Abel L. Peirson, Matteo Perri, Melissa Pesce-Rollins, Maura Pilia, Andrea Possenti, Simonetta Puccetti, Brian D. Ramsey, John Rankin, Oliver J. Roberts, Roger W. Romani, Carmelo Sgrò, Patrick Slane, Paolo Soffitta, Gloria Spandre, Douglas A. Swartz, Toru Tamagawa, Fabrizio Tavecchio, Roberto Taverna, Yuzuru Tawara, Allyn F. Tennant, Nicholas E. Thomas, Alessio Trois, Sergey S. Tsygankov, Roberto Turolla, Jacco Vink, Martin C. Weisskopf, Kinwah Wu, Fei Xie, Silvia Zane

TL;DR

This work uses IXPE to probe the geometry of Cygnus X-3 in the hard X-ray state through energy- and orbital-phase-resolved polarization. The polarization is high and consistent with reflection from an optically thick envelope, implying a narrow funnel geometry that remains stable over roughly a year, even as the source experiences flux variations. The hard-state polarization shows a robust angle near $PA \approx 92^\circ$ with $PD$ around $\sim 21\%$, and orbital-phase modulation supports a geometry tied to the binary orbit and reprocessing in the funnel walls. These findings constrain super-Eddington accretion models in X-ray binaries and reinforce a ULX-like, reflection-dominated scenario for Cyg X-3, while highlighting the need for polarimetric observations across additional accretion states to map the full geometry.

Abstract

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of $ 21.2 \pm 0.4 \% $ and polarization angle of $ 92.2 \pm 0.5^\circ $, well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.

Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

TL;DR

This work uses IXPE to probe the geometry of Cygnus X-3 in the hard X-ray state through energy- and orbital-phase-resolved polarization. The polarization is high and consistent with reflection from an optically thick envelope, implying a narrow funnel geometry that remains stable over roughly a year, even as the source experiences flux variations. The hard-state polarization shows a robust angle near with around , and orbital-phase modulation supports a geometry tied to the binary orbit and reprocessing in the funnel walls. These findings constrain super-Eddington accretion models in X-ray binaries and reinforce a ULX-like, reflection-dominated scenario for Cyg X-3, while highlighting the need for polarimetric observations across additional accretion states to map the full geometry.

Abstract

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of and polarization angle of , well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.

Paper Structure

This paper contains 9 sections, 7 figures.

Table of Contents

  1. Introduction
  2. Observation and Methods
  3. Results
  4. Energy-dependent polarimetry
  5. Polarization in the hard spectral state
  6. Orbital phase-resolved polarimetry
  7. Radio-X-ray correlation
  8. Time-dependent orbit-average analysis
  9. Summary

Figures (7)

  • Figure 1: Normalized Stokes parameters $U/I$ versus $Q/I$ for the IXPE observations of Cyg X-3. HS 2022 in blue, IMS 2022 in yellow, HS 2023 in green and SS 2024 in red color. The error bars are plotted at $1\, \sigma$ CL. In the case of HS 2023, we also plot the change of the polarization properties with the orbital phase in fainter green, numbered 1--12, with bin 1 centered on phase 0. The light green points are shown without error bars, which are on average $\Delta (Q/I) \approx \Delta (U/I)\approx 2\%$ ($1\, \sigma$). The gray grid represents the PD (in %) and PA (in $\degr$).
  • Figure 2: Dependence of PD (top) and PA (bottom) on energy in the 2--8 keV band. HS 2022 in blue squares, IMS 2022 in yellow circles, HS 2023 in green triangles and SS 2024 in red diamonds.
  • Figure 3: Top to bottom: unfolded $EF_E$ (energy flux) spectra of Stokes $I$, $Q$, and $U$, respectively, comparing the spectral shape between HS 2022 (blue) and HS 2023 (green). Spectra are unfolded against a power law with photon index 1.7, for plotting purposes only.
  • Figure 4: IXPE PDs (data points, HS 2022 in blue and HS 2023 in green) with two PD models fitted to the data of the two epochs: a linear change of the PD with energy plus a non-polarized gaussian is shown with the dash-dotted gray line (HS 2022) and the dashed purple line (HS 2023). A constant PD model with a non-polarized Gaussian is shown with the dotted brown line (HS 2022) and the solid orange line (HS 2023).
  • Figure 5: Dependence of the PD (top), PA (middle), and count rate (bottom) on the orbital phase. Each data point represents the mean value for a given orbital phase bin. HS 2022 in blue, IMS 2022 in yellow, HS 2023 in green and SS 2024 in red.
  • ...and 2 more figures