How black holes accrete surrounding matter is a fundamental yet unsolved question in astrophysics. It is generally believed that matter is absorbed into black holes via accretion disks, the state of which depends primarily on the mass-accretion rate. When this rate approaches the critical rate (the Eddington limit), thermal instability is supposed to occur in the inner disk, causing repetitive patterns of large-amplitude X-ray variability (oscillations) on timescales of minutes to hours1. In fact, such oscillations have been observed only in sources with a high mass-accretion rate, such as GRS 1915+105 (refs 2, 3). These large-amplitude, relatively slow timescale, phenomena are thought to have physical origins distinct from those of X-ray or optical variations with small amplitudes and fast timescales (less than about 10 seconds) often observed in other black-hole binaries—for example, XTE J1118+480 (ref. 4) and GX 339−4 (ref. 5). Here we report an extensive multi-colour optical photometric data set of V404 Cygni, an X-ray transient source6 containing a black hole of nine solar masses7 (and a companion star) at a distance of 2.4 kiloparsecs (ref. 8). Our data show that optical oscillations on timescales of 100 seconds to 2.5 hours can occur at mass-accretion rates more than ten times lower than previously thought1. This suggests that the accretion rate is not the critical parameter for inducing inner-disk instabilities. Instead, we propose that a long orbital period is a key condition for these large-amplitude oscillations, because the outer part of the large disk in binaries with long orbital periods will have surface densities too low to maintain sustained mass accretion to the inner part of the disk. The lack of sustained accretion—not the actual rate—would then be the critical factor causing large-amplitude oscillations in long-period systems.
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We acknowledge the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. We also thank the INTEGRAL groups for making the products of the ToO data public online at the INTEGRAL Science Data Centre. Work at ASIAA was supported in part by the thematic research program AS-88-TP-A02. A.S.P., E.D.M. and A.A.V. are grateful to the Russian Science Foundation (grant 15-12-30016) for support. R.Ya.I. is grateful for partial support by the grant RUSTAVELI FR/379/6-300/14. We thank H. Maehara, H. Akazawa, K. Hirosawa and J. Lluis for their optical observations. This work was supported by the Grant-in-Aid “Initiative for High-Dimensional Data-Driven Science through Deepening of Sparse Modeling” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (25120007 TK and 26400228 YU).
The authors declare no competing financial interests.
Reprints and permissions information is available at www.nature.com/reprints.
Extended data figures and tables
Extended Data Figure 1 Optical and X-ray light curves of V404 Cyg during an outburst in 2015 June–July.
a, Overall multi-colour light curves and Swift/BAT light curves. The plotted points are averaged for every 0.67 days. b, An enlarged view of the shaded box in a (the first detection of short-term variations). On BJD 2,457,203, the mean magnitude dropped below V = 17.0. Superimposed on this rapid fading, the amplitude of variations became progressively smaller and smaller. After BJD 2,457,205, the mean magnitude seemed to be constant, and the outburst virtually ended.
Extended Data Figure 2 Additional examples of simultaneous optical and X-ray observations of V404 Cyg in the 2015 outburst.
Data shown in Fig. 3 are excluded. a, b, Main panels, correlations on BJD 2,457,192 (a) and BJD 2,457,200 (b); right panels, Swift/XRT light curves on linear scales. Navy blue error bars, ±1σ.
Extended Data Figure 3 Example of the soft X-ray light curve and spectra during the dip-type oscillation in the 2015 outburst of V404 Cyg.
a, The ~860-s-long Swift/XRT raw light curve (BJD 2,457,194.125–2,457,194.135, ObsID 00031403040) without pile-up correction, same as the X-ray data in Fig. 3a. b, Time-sliced soft X-ray spectra with pile-up correction, in the intervals of T1 to T5 determined in a. The exposures of individual spectra are ~100–300 s. Error bars, ±1σ.
The horizontal axis represents days BJD − 2,429,186, BJD − 2,447,673 and BJD − 2,457,189, respectively. Photographic magnitudes are approximately the same as B band.
Extended Data Figure 5 Power spectral densities of the early stage, the middle stage, and the later stage in the 2015 outburst of V404 Cyg.
Power spectral densities of the fluctuations on BJD 2,457,193 (top, circles), BJD 2,457,196 (middle, triangles) and BJD 2,457,200 (bottom, rectangles). The abscissa and ordinate denote the frequency in Hz and the power in arbitrary units, respectively. For better visualization, the obtained spectrum is multiplied by 8 × 10−4 on BJD 2,457,196 and by 10−4 on BJD 2,457,200. ±1σ error bars obtained from relevant χ2 distributions of the power spectra.
a, b, The intervals shown are BJD 2,457,199.431–2,457,199.446 (a) and BJD 2,457,191.519–2,457,191.524 (b). The optical (V and IC) fluxes are averaged over the intervals; error bars, s.e. The X-ray, U- and UW2-band data are obtained with Swift; error bars, ±1σ. The radio fluxes (open squares) are compiled from the RATAN-600 results at BJD 2,457,199.433 (ref. 68). The red solid and dotted lines show the contribution of emissions from the irradiated disk with Comptonization and from the companion star, respectively. The blue dashed line approximates the radio SED, which is extended to the optical bands for illustrative purposes.
This video shows the “twinkles” of a black hole (short-term and violent variations) in V404 Cyg on June 17 and 18 in 2015 with their image data and light curves. We use the images provided by LCO (Extended Data Table 1). We can see the “twinkles” of a black hole with the naked eyes using a moderate telescope. (MP4 6007 kb)
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Kimura, M., Isogai, K., Kato, T. et al. Repetitive patterns in rapid optical variations in the nearby black-hole binary V404 Cygni. Nature 529, 54–58 (2016) doi:10.1038/nature16452
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