The evolution of rapid optical/X-ray timing correlations in the initial hard state of MAXI J1820+070

We report on a multi-epoch campaign of rapid optical/X-ray timing observations the superbright 2018 outburst MAXI J1820+070, black hole low-mass X-ray binary system. The spanned 80 days in initial hard-state, and were taken with NTT/ULTRACAM GTC/HiPERCAM optical (ugriz filters at time resolutions 8--300 Hz) ISS/NICER X-rays. find (i) growing anti-correlation between lightcurves, (ii) steady, positive correlation an lag 0.2 s (with longer wavelengths) present all epochs, (iii) curious \textit{negative} lags last, softest epoch, wavelengths showing greater more negative lag. To explain these we postulate possible existence two synchrotron-emitting components; compact jet hot flow. In our model, significance decreases over outburst, while flow remains static (thus, relatively, increasing significance). also discuss previously discovered quasi-periodic oscillation note how it creates coherent lags, stronger wavelengths, during least epochs.