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# An extremely powerful long-lived superluminal ejection from the black hole MAXI J1820+070

## Abstract

Black holes in binary systems execute patterns of outburst activity where two characteristic X-ray states are associated with different behaviours observed at radio wavelengths. The hard state is associated with radio emission indicative of a continuously replenished, collimated, relativistic jet, whereas the soft state is rarely associated with radio emission, and never continuously, implying the absence of a quasi-steady jet. Here we report radio observations of the black hole transient MAXI J1820+070 during its 2018 outburst. As the black hole transitioned from the hard to soft state, we observed an isolated radio flare, which, using high-angular-resolution radio observations, we connect with the launch of bipolar relativistic ejecta. This flare occurs as the radio emission of the core jet is suppressed by a factor of over 800. We monitor the evolution of the ejecta over 200 days and to a maximum separation of 10″, during which period it remains detectable due to in situ particle acceleration. Using simultaneous radio observations sensitive to different angular scales, we calculate an accurate estimate of energy content of the approaching ejection. This energy estimate is far larger than that derived from the state transition radio flare, suggesting a systematic underestimate of jet energetics.

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## Data availability

All radio maps used in our analysis are available from the corresponding author on reasonable request. The data used to create the radio–X-ray correlation (Fig. 1) are available in a Source Data file. The AMI-LA data of the radio flare shown in Extended Data Fig. 1 are available in a Source Data file. The authors declare that all other data supporting the findings of this study are available within the paper and its Supplementary Information.

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## Acknowledgements

J.S.B. acknowledges the support of a Science and Technologies Facilities Council Studentship. E.T. acknowledges financial support from the UnivEarthS Labex programme of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). D.A.H.B. acknowledges support by the National Research Foundation. P.A.W. acknowledges support from the NRF and UCT. J.C.A.M.-J. is the recipient of an Australian Research Council Future Fellowship (FT140101082), funded by the Australian government. A.H. acknowledges that this research was supported by a grant from the GIF, the German-Israeli Foundation for Scientific Research and Development. I.H. and D.R.A.W. acknowledge support from the Oxford Hintze Centre for Astrophysical Surveys, which is funded through generous support from the Hintze Family Charitable Foundation. J.M. acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and from the grant RTI2018-096228-B-C31 (MICIU/FEDER, EU).

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Technology. We thank the staff of the Mullard Radio Astronomy Observatory for their invaluable assistance in the commissioning, maintenance and operation of AMI, which is supported by the universities of Cambridge and Oxford. We acknowledge support from the European Research Council under grant ERC-2012-StG-307215 LODESTONE. We thank the Swift team for performing observations promptly on short notice. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC. We acknowledge the use of the Inter-University Institute for Data Intensive Astronomy (IDIA) data-intensive research cloud for data processing. IDIA is a South African university partnership involving the University of Cape Town, the University of Pretoria and the University of the Western Cape. We thank the International Space Science Institute in Bern, Switzerland for support and hospitality for the team meeting ‘Looking at the disc–jet coupling from different angles: inclination dependence of black-hole accretion observables’.

## Author information

Authors

### Contributions

J.S.B. led interpretation of results, wrote a substantial portion of the manuscript, and performed the reduction of the MeerKAT and AMI-LA data. R.P.F. contributed to the interpretation of results and wrote a substantial portion of the manuscript. S.E.M. contributed to the interpretation of results and performed the reduction of the Swift and MAXI X-ray data. J.C.A.M.-J. contributed to the interpretation of results. D.R.A.W., J.M. and R.B. performed the reduction of the eMERLIN data. R.M.P. and J.C.A.M.-J. performed the reduction of the VLA data. J.C.A.M.-J. performed the reduction of the VLBA data. I.H. and E.T. assisted with the reduction of the MeerKAT observational data. D.T., D.A.G., G.R.S., A.J.T., T.D.R. and D.A.H.B. provided useful comments on the manuscript. S.C., J.H., E.G., P.A.W., R.P.A., P.J.G., A.H., A.J.v.d.H., E.G.K., V.A.M., A.R. and R.A.M.J.W. provided useful comments on the manuscript, and were instrumental in the organization and implementation of the ThunderKAT large survey project.

### Corresponding author

Correspondence to J. S. Bright.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Astronomy thanks Michael McCollough and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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

## Extended data

### Extended Data Fig. 1 AMI-LA observations of a state transition radio flare from J1820.

AMI-LA observations of a radio flare which occurred as J1820 transitioned from the hard to soft X-ray state. The blue data points correspond to 30 min of (u,v) amplitudes averaged over all baselines and frequencies. The errors on individual points include a statistical error (calculated from the standard deviation of data within the 30 min bin) and a 5% calibration uncertainty, combined in quadrature. Dotted and dashed lines show exponential fits to the core quenching and the rise of the flare, respectively. We use these to estimate the rise time of the flare, which we take as the time between the intercept of these fits and the peak data point of the flare, as well as its start date. Error bars on data points indicate one sigma uncertainties.

### Extended Data Fig. 2 A VLBA observation of J1820 from MJD 58306.22.

Contours mark $$140\ \mu {\rm{Jy}}\times {(\sqrt{2})}^{n}$$ for n = 3, 4, 5, 6, 7, 8, 9. We mark the position of the core (central red cross; inferred from previous hard state observations) and the measured positions of the approaching (red cross to the right of the core) and receding (red cross to the left of the core) jet from the image. These are given in Supplementary Table 1. The black ellipse in the bottom left corner shows the synthesised beam with a major and minor axis of 0.0009′ and 0.0005′, respectively.

### Extended Data Fig. 3 The radio flux density from the approaching radio ejecta over a 150 d period, starting near our inferred ejection.

As with Fig. 4, with the eMERLIN and VLBA data removed. We fit sections of the light curve with exponential decay functions of the form Fν = AeΔtτ. Data shaded grey are not included in the fitting. The first light curve segment (fast decaying AMI-LA data; MJD 58314 to 58320), is well described ($${\chi }_{\nu }^{2}=1.21$$) by a decay with a characteristic time scale of 6 ± 1d (dashed line). We opt to fit the apparently slower decay (MJD 58324 onward) with a broken exponential function (dotted line). The best fit decay rates are 51 ± 6 d and 21.0 ± 0.9 d, with the break occurring at MJD 58386 ± 4 ($${\chi }_{\nu }^{2}=1.59$$). Error bars on data points indicate one sigma uncertainties.

### Extended Data Fig. 4 The angular separation evolution of the approaching and receding jet components.

The angular separation of the approaching (top panel) and receding (bottom panel) ejections from J1820 with time. We jointly fit both the approaching and receding jet motion with two models. Firstly we assume that both components propagate with ballistic motion and were launched simultaneously. For this case we find μapp = 77 ± 1 mas d−1, μrec = 33 ± 1 mas d−1 and tlaunch = 58305.89 ± 0.02 (Δt = 0.21 ± 0.02) (quantities correspond to the approaching jet velocity, the receding jet velocity and the launch time, respectively). The best fit for this model are shown by the solid black lines in the top and bottom panel. Assuming now as above, but allowing for the proper motion of each component to undergo constant deceleration, we find μapp,0 = 101 ± 3 mas d−1, μrec,0 = 58 ± 6 mas d−1, tlaunch = 58306.03 ± 0.02 (Δt = 0.35 ± 0.02), $${\dot{\mu }}_{{\rm{app}}}=-0.49\pm 0.06\ {\rm{mas}}\ {{\rm{d}}}^{-2}$$ and $${\dot{\mu }}_{{\rm{rec}}}=-0.33\pm 0.07\ {\rm{mas}}\ {{\rm{d}}}^{-2}$$ (quantities correspond to the initial approaching jet velocity, the initial receding jet velocity, the launch time, the deceleration of the approaching jet and the deceleration of the receding jet, respectively). Error bars on data points indicate one sigma uncertainties.

## Supplementary information

### Supplementary Information

Supplementary discussion and Tables 1–4.

## Source data

### Source Data Fig. 1

Data to recreate Fig. 1.

### Source Data Extended Data Fig. 1

Data to recreate Extended Data Fig. 2.

## Rights and permissions

Reprints and Permissions

Bright, J.S., Fender, R.P., Motta, S.E. et al. An extremely powerful long-lived superluminal ejection from the black hole MAXI J1820+070. Nat Astron 4, 697–703 (2020). https://doi.org/10.1038/s41550-020-1023-5

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• DOI: https://doi.org/10.1038/s41550-020-1023-5

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