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

Abstract

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.

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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.

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Acknowledgements

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.

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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.

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Correspondence to H. Takahashi.

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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). https://doi.org/10.1038/s41550-018-0489-x

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