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 hours. 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 339â '4 (ref. 5). Here we report an extensive multi-colour optical photometric data set of V404 Cygni, an X-ray transient source containing a black hole of nine solar masses (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 thought. 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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Acknowledgements 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).
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