1. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, Michigan 48109, USA
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
3. Institute of Astronomy, University of Cambridge, Cambridge CB3 OHA, UK
4. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
5. Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
6. Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Correspondence to: Jon M. Miller¹ Correspondence and requests for materials should be addressed to J.M.M. (Email: jonmm@umich.edu).

Although disk accretion onto compact objects—white dwarfs, neutron stars and black holes—is central to much of high-energy astrophysics, the mechanisms that enable this process have remained observationally difficult to determine. Accretion disks must transfer angular momentum in order for matter to travel radially inward onto the compact object¹. Internal viscosity from magnetic processes^{1, 2, 3, 4} and disk winds⁵ can both in principle transfer angular momentum, but hitherto we lacked evidence that either occurs. Here we report that an X-ray-absorbing wind discovered in an observation of the stellar-mass black hole binary GRO J1655 - 40 (ref. 6) must be powered by a magnetic process that can also drive accretion through the disk. Detailed spectral analysis and modelling of the wind shows that it can only be powered by pressure generated by magnetic viscosity internal to the disk or magnetocentrifugal forces. This result demonstrates that disk accretion onto black holes is a fundamentally magnetic process.

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