Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni


Accretion of matter onto black holes is universally associated with strong radiative feedback1 and powerful outflows2. In particular, black-hole transients3 have outflows whose properties4 are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets5,6. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk7. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10−8 solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks2 imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.

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Figure 1: P Cyg profiles observed during days 2, 6, 7–9 and 10 in He I λ = 5,876 Å.
Figure 2: Trailed spectrum corresponding to data from 19 June (day 2).
Figure 3: Spectral evolution towards the nebular phase.

Change history

  • 01 June 2016

    : A citation to Extended Data Fig. 6 in the Methods was corrected to Extended Data Fig. 5.


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Nine of the spectra of 27 June were taken during the visit of King Felipe VI of Spain to the 10.4-m Gran Telescopio Canarias (GTC); we appreciate the support this visit provides to astrophysical research in Spain. This work is based on observations made with the GTC telescope, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, during both Time Allocation Committee and Director’s Discretionary observing time. We are thankful to the GTC team for the fast response and efficient work throughout the observing campaign. We acknowledge support by the Spanish Ministerio de Economía y competitividad under grants AYA2013-42627 and PSR2015-00397, the Leverhulme Trust Visiting Professorship Grant VP2-2015-046, the International Research Fellowship program of the Japan Society for the Promotion of Science (PE15024), the Bundesministerium für Wirtschaft und Technologie (BMWI/DLR, FKZ 50 OR 1408) and the French Research National Agency’s CHAOS project ANR-12-BS05-0009. The use of the MOLLY software developed by T. R. Marsh is gratefully acknowledged.

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T.M.-D. performed the GTC data analysis and wrote the paper. J.C. contributed to the GTC data analysis and assisted in writing the paper. D.M.S. performed the GTC data reduction and contributed to the GTC data analysis. R.P.F. provided the radio data and contributed to the scientific discussion. M.A.P. performed X-ray analysis and contributed to the scientific discussion. M.L. provided day-12 GTC spectra and assisted in writing the paper. G.P. contributed to the scientific discussion. P.A.C. contributed to the scientific discussion and assisted in writing the paper. K.P.M. performed radio data analysis. J.R. provided part of the INTEGRAL data.

Corresponding author

Correspondence to T. Muñoz-Darias.

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Extended data figures and tables

Extended Data Figure 1 Evolution of the main parameters during the outburst.

Zero time is set to 17 June 00:00 UT. From top to bottom we show hard (25–200 keV) and soft (0.5–10 keV) normalized X-ray count rates, radio flux (~16 GHz), optical continuum flux, He I λ = 5,876 Å equivalent width (EW; positive for absorption) in the range −3,000 km s−1 to 0 km s−1 and Hα equivalent width. The Balmer decrement (black) and Iratio (red) are shown in the bottom panel. In the top panel, X-rays have been normalized to their respective peak at ~LEdd and the time intervals corresponding to the GTC observations have been greyed out for clarity.

Extended Data Figure 2 Trail spectrum showing GTC spectra taken during days 1–6.

P Cyg profiles are apparent in seven transitions of neutral hydrogen (Hα and Hβ) and helium. The strongest are observed in days 1, 2 and 6, being more prominent in the He I λ = 5,876 Å transition. Similar profiles are seen in another five transitions at shorter wavelengths (not shown).

Extended Data Figure 3 Gaussian fits to the P Cyg profiles.

From top to bottom we show the optical flux, terminal (dots) and mean (triangles) velocities, and emission (red dots) and absorption amplitudes (blue dots) of the P Cyg profiles. The two bottom panels result from a Gaussian fitting after subtracting the disk component from the emission (Methods). Terminal velocities in the range VT = 1,500–2,000 km s−1 are observed (see Fig. 2). This method yields VT = 3,000 km s−1 for day 6 (see Fig. 1). The amplitude of the profile is correlated with the optical flux. The similar (if not higher) amplitude of the emission component implies that the wind has a large covering factor. Error bars indicate the standard error of the mean.

Extended Data Figure 4 Evolution of the Hα emission line.

The top panel shows the evolution of the centroid of the Hα line. Positive velocity values are due to line asymmetries by blue absorption and red emission. The bottom panel shows the V/R parameter (the emission line is symmetric if V/R = 1) showing the same phenomena. This strongly suggests the presence of continuous outflows from the outer disk along the whole outburst. Error bars indicate the standard deviation of measurements within each observing window.

Extended Data Figure 5 BD evolution through the nebular phase.

From day 10, the BD is observed to increase sharply, reaching ~5 on day 11 and ~6 on day 12 (see Methods). Dotted lines join observations consecutive in time. Error bars indicate the standard error of the mean.

Extended Data Table 1 Log of the GTC observations

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Muñoz-Darias, T., Casares, J., Sánchez, D. et al. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni. Nature 534, 75–78 (2016).

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