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The black-hole X-ray transient MAXI J1820+07 (=ASSASN-18ey) discovered inMarch 2018 was one of the optically brightest ever seen, which has resulted invery detailed optical outburst light-curves being obtained. We combine themhere with X-ray and radio light-curves to show the major geometric changes thesource undergoes. We present a detailed temporal analysis that reveals thepresence of remarkably high amplitude (>0.5 mag) modulations, which evolve fromthe superhump (16.87 h) period towards the presumed orbital (16.45 h) period.These modulations appear ~87d after the outburst began, and follow theSwift/BAT hard X-ray light-curve, which peaks 4 days before the radio flare andjet ejection, when the source undergoes a rapid hard to soft state transition.The optical modulation then moves closer to the orbital period, with a lightcurve peak that drifts slowly in orbital phase from ~0.8 to ~0.3 during thesoft state. We propose that the unprecedentedly large amplitude modulationrequires a warp in the disc in order to provide a large enough radiating area,and for the warp to be irradiation-driven. Its sudden turn-on implies a changein the inner disc geometry that raises the hard X-ray emitting component to aheight where it can illuminate the warped outer disc regions.

Large optical modulations during 2018 outburst of MAXI J1820+070 reveal evolution of warped accretion disc through X-ray state change