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Indeed some pretty esciting data from Nick. Incredible to think we are looking at the secondary effects of a black hole.
Paul, I am not sure I know what is causing these variations. Much of the work on V404 Cyg has been whilst it was in quiescence, where large variations, or flares, are also seen. One idea is that the flares are caused by local magnetic reconnection events in or above the accretion disc. The theory goes that a dynamo mechanism, driven by the strong shear produced by differential rotation, operates in accretion discs, and regions of oppositely directed magnetic fields develop within the disc or between the disc and corona and reconnect explosively. There’s more discussion in the MNRAS paper: http://cdsbib.u-strasbg.fr/cgi-bin/cdsbib?2003MNRAS.346.1116S
However, maybe in outburst it’s something else? I am sure that the current outburst will reveal more – and much of the new data will be contributed by amateur astronomers!
I hope others will get a chance to see such a rare outburst of this peculiar system before it fades.