Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system


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.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Multiwavelength light curves and timing correlations of V404 Cygni on 25 June 2015.
Fig. 2: X-ray evolution of V404 Cygni leading to outburst peak.
Fig. 3: Schematic of the post-transition accretion and jet geometry of V404 Cygni.

Similar content being viewed by others


  1. Blandford, R. D. & Znajek, R. L. Electromagnetic extraction of energy from Kerr black holes. Mon. Not. R. Astron. Soc. 179, 433–456 (1977).

    Article  ADS  Google Scholar 

  2. Blandford, R. D. & Payne, D. G. Hydrodynamic flows from accretion discs and the production of radio jets. Mon. Not. R. Astron. Soc. 199, 883–903 (1982).

    Article  ADS  MATH  Google Scholar 

  3. Meier, D. L., Koide, S. & Uchida, Y. Magnetohydrodynamic production of relativistic jets. Science 291, 84–92 (2001).

    Article  ADS  Google Scholar 

  4. Merloni, A. et al. Magnetic flares and the optical variability of the X-ray transient XTE J1118+480. Mon. Not. R. Astron. Soc. 318, L15–L19 (2000).

    Article  ADS  Google Scholar 

  5. Malzac, J. et al. Jet–disc coupling through a common energy reservoir in the black hole XTE J1118+480. Mon. Not. R. Astron. Soc. 351, 253–264 (2004).

    Article  ADS  Google Scholar 

  6. Yuan, F. et al. An accretion-jet model for black hole binaries: interpreting the spectral and timing features of XTE J1118+480. Astrophys. J. 620, 905–914 (2005).

    Article  ADS  Google Scholar 

  7. Veledina, A. et al. Hot accretion flow in black hole binaries: a link connecting X-rays to the infrared. Mon. Not. R. Astron. Soc. 430, 3196–3212 (2013).

    Article  ADS  Google Scholar 

  8. Gandhi, P. et al. Rapid optical and X-ray timing observations of GX 339–4: flux correlations at the onset of a low/hard state. Mon. Not. R. Astron. Soc. 390, L29–L33 (2008).

    Article  ADS  Google Scholar 

  9. Markoff, S. et al. 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).

    Article  ADS  Google Scholar 

  10. Malzac, J. The spectral energy distribution of compact jets powered by internal shocks. Mon. Not. R. Astron. Soc. 443, 299–317 (2014).

    Article  ADS  Google Scholar 

  11. Polko, P. et al. Linking accretion flow and particle acceleration in jets: II. Self-similar jet models with full relativistic MHD gravitational mass. Mon. Not. R. Astron. Soc. 438, 959–970 (2014).

    Article  ADS  Google Scholar 

  12. Marscher, A. et al. The inner jet of an active galactic nucleus as revealed by a radio-to-gamma-ray outburst. Nature 452, 966–969 (2008).

    Article  ADS  Google Scholar 

  13. Cohen, M. H. et al. Studies of the jet in Bl Lacertae. I. Recollimation shock and moving emission features. Astrophys. J. 787, 151–160 (2014).

    Article  ADS  Google Scholar 

  14. Casella, P. et al. Fast infrared variability from a relativistic jet in GX 339–4. Mon. Not. R. Astron. Soc. 404, L21–L25 (2010).

    Article  ADS  Google Scholar 

  15. Gandhi, P. et al. Furiously fast and red: sub-second optical flaring in V404 Cyg during the 2015 outburst peak. Mon. Not. R. Astron. Soc. 459, 554–572 (2016).

    Article  ADS  Google Scholar 

  16. Walton, D. J. et al. Living on a flare: relativistic reflection in V404 Cyg observed by NuSTAR during its summer 2015 outburst. Astrophys. J. 839, 110–132 (2017).

    Article  ADS  Google Scholar 

  17. Loh, A. et al. High-energy gamma-ray observations of the accreting black hole V404 Cygni during its 2015 June outburst. Mon. Not. R. Astron. Soc. 462, L111–L115 (2016).

    Article  ADS  Google Scholar 

  18. Gandhi, P. et al. Correlated optical and X-ray variability in V404 Cyg. Astron. Telegr. 7727 (2015).

  19. Rodriguez, J. et al. Correlated optical, X-ray, and γ-ray flaring activity seen with INTEGRAL during the 2015 outburst of V404 Cygni. Astron. Astrophys. 581, L9–L13 (2015).

    Article  ADS  Google Scholar 

  20. Durant, M. et al. High time resolution optical/X-ray cross-correlations for X-ray binaries: anticorrelation and rapid variability. Mon. Not. R. Astron. Soc. 410, 2329–2338 (2011).

    Article  ADS  Google Scholar 

  21. Khargharia, J., Froning, C. S. & Robinson, E. L. Near-infrared spectroscopy of low-mass X-ray binaries: accretion disk contamination and compact object mass determination in V404 Cyg and Cen X-4. Astrophys. J. 716, 1105–1117 (2010).

    Article  ADS  Google Scholar 

  22. Shahbaz, T. et al. Evidence for magnetic field compression in shocks within the jet of V404 Cyg. Mon. Not. R. Astron. Soc. 463, 1822–1830 (2016).

    Article  ADS  Google Scholar 

  23. Motta, S. et al. The black hole binary V404 Cygni: an obscured AGN analogue. Mon. Not. R. Astron. Soc. 468, 981–993 (2017).

    Article  ADS  Google Scholar 

  24. Jamil, O., Fender, R. P. & Kaiser, C. R. iShocks: X-ray binary jets with an internal shock model. Mon. Not. R. Astron. Soc. 401, 394–404 (2010).

    Article  ADS  Google Scholar 

  25. Falcke, H., Körding, E. & Markoff, S. A scheme to unify low power accreting black holes. Astron. Astrophys. 414, 895–903 (2004).

    Article  ADS  Google Scholar 

  26. Merloni, A. et al. A fundamental plane of black hole activity. Mon. Not. R. Astron. Soc. 345, 1057–1076 (2003).

    Article  ADS  Google Scholar 

  27. Kanbach, G. et al. Correlated fast X-ray and optical variability in the black-hole candidate XTE J1118+480. Nature 414, 180–182 (2001).

    Article  ADS  Google Scholar 

  28. Kimura, M. et al. Repetitive patterns in rapid optical variations in the nearby black-hole binary V404 Cygni. Nature 529, 54–58 (2016).

    Article  ADS  Google Scholar 

  29. Dhillon, V. S. et al. ULTRACAM: an ultrafast, triple-beam CCD camera for high-speed astrophysics. Mon. Not. R. Astron. Soc. 378, 825–840 (2007).

    Article  ADS  Google Scholar 

  30. Harrison, F. et al. The Nuclear Spectroscopic Telescope Array (NuSTAR) high-energy X-ray mission. Astrophys. J. 770, 103–131 (2013).

    Article  ADS  Google Scholar 

  31. Bachetti, M. et al. No time for dead time: timing analysis of bright black hole binaries with NuSTAR. Astrophys. J. 800, 109–120 (2015).

    Article  ADS  Google Scholar 

  32. Blackburn, J. K. FTOOLS: A FITS data processing and analysis software package. Astr. Soc. P. 77, 367–370 (1995).

    ADS  Google Scholar 

  33. Zwart, J. T. L. et al. The Arcminute Microkelvin Imager. Mon. Not. R. Astron. Soc. 391, 1545–1558 (2008).

    Article  ADS  Google Scholar 

  34. Winkler, C. et al. The INTEGRAL mission. Astron. Astrophys. 411, L1–L6 (2003).

    Article  ADS  Google Scholar 

  35. Kuulkers, E. INTEGRAL observations of V404 Cyg (GS 2023+338): public data products. Astron. Telegr. 7758 (2015).

  36. Ubertini, P. et al. IBIS: the imager on-board INTEGRAL. Astron. Astrophys. 411, L131–L139 (2003).

    Article  ADS  Google Scholar 

  37. Edelson, R. A. & Krolik, J. H. The discrete correlation function: a new method for analyzing unevenly sampled variability data. Astrophys. J. 333, 646–659 (1988).

    Article  ADS  Google Scholar 

  38. Welsh, W. F. On the reliability of cross-correlation function lag determinations in active galactic nuclei. Publ. Astron. Soc. Pac. 111, 1347–1366 (1999).

    Article  ADS  Google Scholar 

  39. Koratkar, A. P. & Gaskell, C. M. Structure and kinematics of the broad-line regions in active galaxies from IUE variability data. Astrophys. J. Suppl. 75, 719–750 (1991).

    Article  ADS  Google Scholar 

  40. Peterson, B. M. et al. On uncertainties in cross-correlation lags and the reality of wavelength-dependent continuum lags in active galactic nuclei. Publ. Astron. Soc. Pacific. 110, 660–670 (1998).

    Article  ADS  Google Scholar 

Download references


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.

Author information

Authors and Affiliations



P.G. wrote the ULTRACAM proposal, analysed the data and wrote the paper. The ULTRACAM observations were coordinated and carried out by L.K.H., S.P.L., V.S.D. and T.R.M. The X-ray observations were proposed by D.J.W., coordinated by D.S., J.A.T. and F.A.H., and the timing data analysed by M.B. Radio data were obtained and analysed by R.P.F. and K.M. INTEGRAL data were arranged by E.K. The remaining authors provided insight into jet physics constraints (S.M., J.M., C.C.), cross-correlation analyses (P.C., R.I.H., C.K., C.F., M.P., F.V.) and placing the source in context (D.A., J.C., P.A.C., D.M.R., F.R., A.W.S.). All authors read and commented on multiple versions of the manuscript.

Corresponding author

Correspondence to P. Gandhi.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Additional information

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

Electronic supplementary material

Supplementary Information

5 supplementary figures, 7 sections, 47 references

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Gandhi, P., Bachetti, M., Dhillon, V.S. et al. An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system. Nat Astron 1, 859–864 (2017).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing