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Strong disk winds traced throughout outbursts in black-hole X-ray binaries


Recurring outbursts associated with matter flowing onto compact stellar remnants (such as black holes, neutron stars and white dwarfs) in close binary systems provide a way of constraining the poorly understood accretion process. The light curves of these outbursts are shaped by the efficiency of angular-momentum (and thus mass) transport in the accretion disks, which has traditionally been encoded in a viscosity parameter, α. Numerical simulations1,2,3 of the magneto-rotational instability that is believed to be the physical mechanism behind this transport yield values of α of roughly 0.1–0.2, consistent with values determined from observations of accreting white dwarfs4. Equivalent viscosity parameters have hitherto not been estimated for disks around neutron stars or black holes. Here we report the results of an analysis of archival X-ray light curves of 21 outbursts in black-hole X-ray binaries. By applying a Bayesian approach to a model of accretion, we determine corresponding values of α of around 0.2–1.0. These high values may be interpreted as an indication either of a very high intrinsic rate of angular-momentum transport in the disk, which could be sustained by the magneto-rotational instability only if a large-scale magnetic field threads the disk5,6,7, or that mass is being lost from the disk through substantial outflows, which strongly shape the outburst in the black-hole X-ray binary. The lack of correlation between our estimates of α and the accretion state of the binaries implies that such outflows can remove a substantial fraction of the disk mass in all accretion states and therefore suggests that the outflows correspond to magnetically driven disk winds rather than thermally driven ones, which require specific radiative conditions8.

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Figure 1: Example light curves of outbursts in low-mass X-ray binaries.
Figure 2: Characterization of the mass-transport process in accretion disks.
Figure 3: Toy model of a disk ‘wind’.


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B.E.T. thanks participants of the ‘disks17: Confronting MHD Theories of Accretion Disks with Observations’ programme, held at the Kavli Institute for Theoretical Physics (KITP), for their feedback and comments on this project, especially A. Veledina for advice regarding the analysis of the X-ray light curves and P. Charles for comments on the manuscript. B.E.T., G.R.S. and C.O.H. acknowledge support by NSERC Discovery Grants, and C.O.H. by a Discovery Accelerator Supplement. This research was supported in part by the National Science Foundation under grant number NSF PHY-1125915, via support for KITP. J.-P.L. acknowledges support by the Polish National Science Centre OPUS grant 2015/19/B/ST9/01099. J.-P.L. and G.D. also acknowledge support from the French Space Agency CNES. This research has made use of data, software, and/or web tools obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and of the Smithsonian Astrophysical Observatory’s High Energy Astrophysics Division, data supplied by the UK Swift Science Data Centre at the University of Leicester, and data provided by RIKEN, JAXA and the MAXI team. This work has also made extensive use of NASA’s Astrophysics Data System (ADS).

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Authors and Affiliations



B.E.T. performed the analysis of the X-ray data, wrote the Markov chain Monte Carlo light-curve-fitting algorithm and performed the light-curve fitting, built the Bayesian hierarchical methodology and wrote the paper. J.-P.L. helped to formulate the analytical version of the irradiated-disk-instability model that was fitted to the X-ray light curves, contributed to the interpretation of the data and assisted in writing the discussion in the paper. C.O.H. assisted in the analysis of the X-ray data and the light-curve-fitting process, and contributed to the interpretation of the data. G.D. contributed to the interpretation of the data and assisted in writing the discussion in the paper. G.R.S. assisted in writing the paper and contributed to the interpretation of the data.

Corresponding author

Correspondence to B. E. Tetarenko.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks D. Proga and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Schematic light curve for an outburst of a low-mass X-ray binary system.

The profile shown corresponds to the light curve predicted by the (irradiated) disk-instability model for an outbursting irradiated disk. τe and τl represent the timescales of the exponential (viscous) and linear (irradiation-controlled) decay stages in the light curve, respectively. The time and flux at which the transition between the viscous and exponential stages of the decay occurs (marking the point at which the temperature in the outer disk drops below the ionization temperature of hydrogen) are represented by tbreak and ft, respectively. The inset shows the same light-curve profile on a linear scale.

Extended Data Table 1 Binary orbital parameters for our Galactic black-hole low-mass X-ray binary source sample
Extended Data Table 2 Quantities derived to describe the mass-transport process in the accretion disks of outbursting low-mass X-ray binaries

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Tetarenko, B., Lasota, JP., Heinke, C. et al. Strong disk winds traced throughout outbursts in black-hole X-ray binaries. Nature 554, 69–72 (2018).

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