Optical and X-ray timing analysis of the 2018-2020 outburst and rebrightening of the black-hole transient MAXI J1820+070

Abstract: We report the results of a comprehensive analysis of the multiwavelength (in optical and X-rays) and multitimescale (from months to tenths of a second) variability of the 2018-2020 outburst of the black hole transient MAXI J1820+070. During the first outburst episode, a detailed analysis of the optical photometry shows a periodicity that evolves over time and stabilises at a frequency of $1.4517(1)$ $1/d$ ($\sim0.5\%$ longer than the orbital period). This super-orbital modulation is also seen in the X-rays for a few days soon after the transition to the high-soft state. We also observed optical Quasi-Periodic Oscillations (QPOs), which correspond to some of the QPOs observed in X-rays at three different epochs when the source was in the low-hard state. In two epochs, optical QPOs with a centroid consistent with half the frequency of the most prominent X-ray QPO can be seen. If the lowest modulation frequency is the one observed in the optical, the characteristic precession frequency of MAXI J1820+070 is lower than that inferred from the `fundamental' QPO in the X-rays. Assuming that QPOs can be generated by Lense-Thirring precession, we calculate the spin of the black hole in the case where the fundamental precession frequency is tracked by the optical emission. We find a relatively slowly spinning black hole with a spin parameter $\lesssim 0.15$. The super-orbital optical and X-ray modulations observed after the disappearance of the QPOs may be triggered by the self-irradiation of the outer disc by a standard inner disc truncated at a few gravitational radii.