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

We report the results of a comprehensive analysis of the multi-wavelength (in optical and X-rays) and multi-timescale (from months to tenths of a second) variability of the 2018–2020 outburst of the black hole transient MAXI J1820+070. A detailed analysis of the optical photometry revealed a periodicity that evolves over time during the first outburst episode and stabilises at a frequency of 1.4517(1) 1/d (∼0.5% longer than the orbital period). This super-orbital modulation is also seen in the X-rays for a few days shortly after the transition to the high-soft state. We also observed optical quasi-periodic oscillations (QPOs) that 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 calculated 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 of ≲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.