Relativistic plasma jets are observed in many systems that host accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched1,2,3. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot easily be disentangled from other accreting components. Here, we show that rapid optical flux variations from an accreting Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by about 0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these subsecond optical variations has hitherto been controversial4,5,6,7,8. Not only does our work strongly support a jet origin for the optical variations but it also sets a characteristic elevation of ≲103 Schwarzschild radii for the main inner optical emission zone above the black hole9, constraining both internal shock10 and magnetohydrodynamic11 models. Similarities with blazars12,13 suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black-hole binaries show very similar optical lags8,14,15, so this size scale may be a defining feature of such systems.
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This research has made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA), as well as the High Energy Astrophysics Science Archive Research Center. P.G. thanks the Science and Technology Facilities Council (STFC) for support (grant reference ST/J003697/2). ULTRACAM and V.S.D. are supported by STFC grant ST/M001350/1. P.G. thanks C.B. Markwardt, C.M. Boon, A.B. Hill, M. Fiocchi, K. Forster, A. Zoghbi and T. Muñoz-Darias for help and discussions. J.C. acknowledges financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) under the 2015 Severo Ochoa Program MINECO SEV-2015-0548, and to the Leverhulme Trust through grant VP2-2015-04. T.R.M. acknowledges STFC (ST/L000733/1). J.M. acknowledges financial support from the French National Research Agency (CHAOS project ANR-12-BS05-0009), and D.A. thanks the Royal Society. S.M. acknowledges support from Netherlands Organisation for Scientific Research (NWO) VICI grant no. 639.043.513. We thank P. Wallace for use of his SLA C library. P.A.C. is grateful to the Leverhulme Trust for the award of a Leverhulme Emeritus Fellowship. Part of this research was supported by the UK-India UKIERI/UGC Thematic Partnership grants UGC 2014-15/02 and IND/CONT/E/14-15/355. This work profited from discussions carried out during a meeting organized at the International Space Science Institute (ISSI) Beijing by T. Belloni andD. Bhattacharya.