Paice, J.A., Gandhi, Poshak, Shahbaz, T., Veledina, A., Malzac, J., Charles, P.A., Rajwade, K., Dhillon, V.S., Littlefair, S.P., Marsh, T.R., Uttley, P., Vincentelli, F.M. and Misra, R. (2021) The evolution of rapid optical/X-ray timing correlations in the initial hard state of MAXIJ1820+070. Monthly Notices of the Royal Astronomical Society. (In Press)
Abstract
We report on a multi-epoch campaign of rapid optical/X-ray timing observations of the superbright 2018 outburst of MAXI J1820+070, a black hole low-mass X-ray binary system. The observations spanned 80 days in the initial hard-state, and were taken with NTT/ULTRACAM and GTC/HiPERCAM in the optical (usgsrsiszs filters at time resolutions of 8{300 Hz) and with ISS/NICER in X-rays. We find (i) a growing anti-correlation between the optical and X-ray lightcurves, (ii) a steady, positive correlation at an optical lag of 0.2 s (with a longer lag at longer wavelengths) present in all epochs, and (iii) a curious positive correlation at negative optical lags in the last, X-ray softest epoch, with longer wavelengths showing a greater correlation and a more negative lag. To explain these we postulate the possible existence of two synchrotron-emitting components; a compact jet and a hot flow. In our model, the significance of the jet decreases over the outburst, while the hot flow remains static (thus, relatively, increasing in significance). We also discuss a previously discovered quasi-periodic oscillation and note how it creates coherent optical time lags, stronger at longer wavelengths, during at least two epochs.
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