Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry


Black hole binary (BHB) systems comprise a stellar-mass black hole and a closely orbiting companion star. Matter is transferred from the companion to the black hole, forming an accretion disk, corona and jet structures. The resulting release of gravitational energy leads to the emission of X-rays1. The radiation is affected by special/general relativistic effects, and can serve as a probe for the properties of the black hole and surrounding environment, if the accretion geometry is properly identified. Two competing models describe the disk–corona geometry for the hard spectral state of BHBs, based on spectral and timing measurements2,3. Measuring the polarization of hard X-rays reflected from the disk allows the geometry to be determined. The extent of the corona differs between the two models, affecting the strength of the relativistic effects (such as enhancement of the polarization fraction and rotation of the polarization angle). Here, we report observational results on the linear polarization of hard X-ray emission (19–181 keV) from a BHB, Cygnus X-14, in the hard state. The low polarization fraction, <8.6% (upper limit at a 90% confidence level), and the alignment of the polarization angle with the jet axis show that the dominant emission is not influenced by strong gravity. When considered together with existing spectral and timing data, our result reveals that the accretion corona is either an extended structure, or is located far from the black hole in the hard state of Cygnus X-1.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Comparison of the ‘lamp-post corona’ and ‘extended corona’ models describing hard X-ray emissions from black hole binaries.
Fig. 2: Posterior density and credibility regions of PF and PA measured by PoGO+ after background subtraction.
Fig. 3: Comparison of PoGO+ polarimetric and other spectral results for the lamp-post corona model.


  1. 1.

    Remillard, R. A. & McClintock, J. E. X-ray properties of black-hole binaries. Annu. Rev. Astron. Astr. 44, 49–92 (2006).

    ADS  Article  Google Scholar 

  2. 2.

    Makishima, K. et al. Suzaku results on Cygnus X-1 in the low/hard state. Publ. Astron. Soc. Jpn 60, 585–604 (2008).

    ADS  Article  Google Scholar 

  3. 3.

    Fabian, A. C. et al. On the determination of the spin of the black hole in Cyg X-1 from X-ray reflection spectra. Mon. Not. R. Astron. Soc. 424, 217–223 (2012).

    ADS  Article  Google Scholar 

  4. 4.

    Webster, B. L. & Murdin, P. Cygnus X-1-a spectroscopic binary with a heavy companion? Nature 235, 37–28 (1972).

    ADS  Article  Google Scholar 

  5. 5.

    Miniutti, G. & Fabian, A. C. A light bending model for the X-ray temporal and spectral properties of accreting black holes. Mon. Not. R. Astron. Soc. 349, 1435–1448 (2004).

    ADS  Article  Google Scholar 

  6. 6.

    Frontera, F. Spectral and temporal behavior of the black hole candidate XTE J1118+480 as observed with BeppoSAX. Astrophys. J. 592, 1110–1118 (2003).

    ADS  Article  Google Scholar 

  7. 7.

    Basak, R., Zdziarski, A. A., Parker, M. & Islam, N. Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state: evidence for a truncated disc geometry. Mon. Not. R. Astron. Soc. 472, 4220–4232 (2017).

    ADS  Article  Google Scholar 

  8. 8.

    Lightman, A. P. & Shapiro, S. L. Spectrum and polarization of X-rays from accretion disks around black holes. Astrophys. J. Lett. 198, L73–L75 (1975).

    ADS  Article  Google Scholar 

  9. 9.

    Chauvin, M. et al. Calibration and performance studies of the balloon-borne hard X-ray polarimeter PoGO+. Nucl. Instrum. Meth. A 859, 125–133 (2017).

    ADS  Article  Google Scholar 

  10. 10.

    Stirling, A. M. et al. A relativistic jet from Cygnus X-1 in the low/hard X-ray state. Mon. Not. R. Astron. Soc. 327, 1273–1278 (2001).

    ADS  Article  Google Scholar 

  11. 11.

    Fender, R. P. A transient relativistic radio jet from Cygnus X-1. Mon. Not. R. Astron. Soc. 369, 603–607 (2006).

    ADS  Article  Google Scholar 

  12. 12.

    Reid, M. J., McClintock, J. E., Narayan, R., Gou, L., Remillard, R. A. & Orosz, J. A. The trigonometric parallax of Cygnus X-1. Astrophys. J. 742, 83–87 (2011).

    ADS  Article  Google Scholar 

  13. 13.

    Walton, D. J. et al. The soft state of Cygnus X-1 observed with NuSTAR: a variable corona and a stable inner disk. Astrophys. J. 826, 87–99 (2016).

    ADS  Article  Google Scholar 

  14. 14.

    Chandrasekhar, S. Radiative Transfer (Dover Publications, New York, 1960).

  15. 15.

    Dovciak, M., Muleri, F., Goosmann, R. W., Karas, V. & Matt, G. Light-bending scenario for accreting black holes in X-ray polarimetry. Astrophys. J. 731, 75–89 (2011).

    ADS  Article  Google Scholar 

  16. 16.

    Dovciak, M., Muleri, F., Goosmann, R. W., Karas, V. & Matt, G. Polarization in lamp-post model of black-hole accretion discs. J. Phys. Conf. Ser. 372, 012056 (2012).

    Article  Google Scholar 

  17. 17.

    Schnittman, J. D. & Krolik, J. H. X-ray polarization from accreting black holes: coronal emission. Astrophys. J. 712, 908–924 (2010).

    ADS  Article  Google Scholar 

  18. 18.

    Beloborodov, A. M. Plasma ejection from magnetic flares and the X-ray spectrum of Cygnus X-1. Astrophys. J. Lett. 510, L123–L126 (1999).

    ADS  Article  Google Scholar 

  19. 19.

    Done, C., Gierlinski, M. & Kubota, A. Modelling the behaviour of accretion flows in X-ray binaries. Everything you always wanted to know about accretion but were afraid to ask. Astron Astrophys Rev. 15, 1–66 (2007).

    ADS  Article  Google Scholar 

  20. 20.

    Nowak, M. et al. Corona, jet and relativistic line models for Suzaku/RXTE/Chandra-HETG observations of Cygnus X-1 hard state. Astrophys. J. 728, 13–41 (2011).

    ADS  Article  Google Scholar 

  21. 21.

    Niedzwiecki, A., Zdziarski, A. A. & Szanecki, M. On the lamppost model of accreting black holes. Astrophys. J. Lett. 821, L1–L5 (2016).

    ADS  Article  Google Scholar 

  22. 22.

    De Marco, B. et al. Evolution of the reverberation lag in GX 339-4 at the end of an outburst. Mon. Not. R. Astron. Soc. 471, 1475–1487 (2017).

    ADS  Article  Google Scholar 

  23. 23.

    Ohsuga, K., Mineshige, S., Mori, M. & Kato, Y. Global radiation-magnetohydrodynamic simulations of black-hole accretion flow and outflow: unified model of three states. Publ. Astron. Soc. Jpn 61, L7–L11 (2009).

    ADS  Article  Google Scholar 

  24. 24.

    Miller, J. M. et al. New constrains on the black hole low/hard state inner accretion flow with NuSTAR. Astrophys. J. Lett. 799, L6–L15 (2015).

    ADS  Article  Google Scholar 

  25. 25.

    Reynolds, C. S. Measuring black hole spin using X-ray reflection spectroscopy. Space Sci. Rev. 183, 277–294 (2014).

    ADS  Article  Google Scholar 

  26. 26.

    Merloni, A., Heinz, S. & di Matteo, T. A fundamentral plane of black hole activity. Mon. Not. R. Astron. Soc. 345, 1057–1076 (2003).

    ADS  Article  Google Scholar 

  27. 27.

    Zanin, R. et al. Gamma rays detected from Cygnus X-1 with likely jet origin. Astron. Astrophys. 596, A55 (2016).

    Article  Google Scholar 

  28. 28.

    Albert, J. et al. Very high energy gamma-ray radiation from the stellar mass black hole binary Cygnus X-1. Astrophys. J. Lett. 665, L51–L54 (2007).

    ADS  Article  Google Scholar 

  29. 29.

    Orosz, J. A. The mass of the black hole in Cygnus X-1. Astrophys. J. 742, 84–93 (2011).

    ADS  Article  Google Scholar 

  30. 30.

    Ziolkowski, J. Determination of the masses of the components of the HDE 226868/Cyg X-1 binary system. Mon. Not. R. Astron. Soc. Lett. 440, L61–L65 (2014).

    ADS  Article  Google Scholar 

  31. 31.

    Agostinell, S. et al. Geant4-a simulation toolkit. Nucl. Instrum. Meth. A 506, 250–303 (2003).

    ADS  Article  Google Scholar 

  32. 32.

    Matsuoka, M. et al. The MAXI mission on the ISS: science and instruments for monitoring all-sky X-ray images. Publ. Astron. Soc. Jpn 61, 999–1010 (2009).

    ADS  Article  Google Scholar 

  33. 33.

    Krimm, H. et al. The Swift/BAT hard X-ray transient monitor. Astrophys. J. Suppl. S. 209, 14–46 (2013).

    ADS  Article  Google Scholar 

  34. 34.

    Mitsuda, K. et al. The X-ray observatory Suzaku. Publ. Astron. Soc. Jpn 59, S1–S7 (2007).

    Article  Google Scholar 

  35. 35.

    Chauvin, M. et al. Shedding new light on the Crab with polarized X-rays. Sci. Rep. 7, 7816–7821 (2017).

    ADS  Article  Google Scholar 

  36. 36.

    Russell, D. M. & Shahbaz, T. The multiwavelength polarizatoin of Cygnus X-1. Mon. Not. R. Astron. Soc. 43, 2083–2096 (2014).

    ADS  Article  Google Scholar 

  37. 37.

    Long, K. S., Chanan, G. A. & Novick, R. The X-ray polarization of Cygnus sources. Astrophys. J. 238, 710–716 (1980).

    ADS  Article  Google Scholar 

  38. 38.

    Laurent, P. et al. Polarized gamma-ray emission from the Galactic black hole Cygnus X-1. Science 332, 438–439 (2011).

    ADS  Article  Google Scholar 

  39. 39.

    Jourdain, E., Roques, J. P., Chauvin, M. & Clark, D. J. Separation of two contributions to the high energy emission of Cygnus X-1: polarization measurements with INTEGRAL SPI. Astrophys. J. 761, 27–34 (2012).

    ADS  Article  Google Scholar 

  40. 40.

    Markoff, S., Falcke, H. & Fender, R. A jet model for the broadband spectrum of XTE J1118+480: synchrotron emission from radio to X-rays in the Low/Hard spectral state. Astron. Astrophys. 372, L25–L28 (2001).

    ADS  Article  Google Scholar 

  41. 41.

    Matt, G. X-ray polarization properties of a centrally illuminated accretion disc. Mon. Not. R. Astron. Soc. 260, 663–674 (1993).

    ADS  Article  Google Scholar 

Download references


This research was supported in Sweden by The Swedish National Space Board, The Knut and Alice Wallenberg Foundation, and The Swedish Research Council. In Japan, support was provided by Japan Society for Promotion of Science and ISAS/JAXA. SSC are thanked for providing expert mission support and launch services at Esrange Space Centre. DST Control developed the PoGO+ attitude control system under the leadership of J.-E. Strömberg. Contributions from past Collaboration members and students are acknowledged. In particular, we thank M. Kole, E. Moretti, G. Olofsson and S. Rydström for their important contributions to the PoGOLite Pathfinder mission from which PoGO+ was developed.

Author information




M.C., H-G.F., M.F., M.J., T.Kam., J.K., T.Kaw., M.K., V.M., T.M., N.O., T.S., H.T., H.Tak., N.U. and M.P. contributed to the development of the PoGO+ mission concept and/or construction and testing of polarimeter hardware and software. Observations were conducted by M.C., H-G.F., M.F., M.K., V.M., T.S., H.Tak., N.U. and M.P. Data reduction and analysis was performed by M.C., M.F., M.K., V.M., H.Tak. and M.P. The manuscript was prepared by M.F., M.K., V.M., H.Tak. and M.P. The mission principal investigator is M.P.

Corresponding author

Correspondence to H. Takahashi.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figure 1–12, Supplementary Table 1.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Chauvin, M., Florén, H., Friis, M. et al. Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry. Nat Astron 2, 652–655 (2018).

Download citation

Further reading