Observations of V404 Cygni, an X-ray transient containing a black hole of nine solar masses and a companion star, show that optical oscillations on timescales of 100 seconds to 2.5 hours can occur at mass-accretion rates at least 10 times slower than previously thought, suggesting that the accretion rate is not the critical parameter for inducing inner-disk instabilities. The authors propose that a long orbital period is a key condition, because the outer part of the disc will have surface densities too low to maintain sustained mass accretion to the inner part of the disc. The lack of sustained accretion — rather than the actual rate — would then be the critical factor causing violent oscillations in long-period systems.
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