2025A&A...704A.198B

We present a study of the evolution of the minute-timescale optical variability and of the optical spectroscopic signatures of outflows in the black hole X-ray binary MAXI J1820+070 during its main 2018 outburst and subsequent re-brightenings. Multi-filter, minute-cadence optical light curves were obtained with the Las Cumbres Observatory network and the Al Sadeem Observatory (UAE) over 2018-2020, complemented by archival X-ray data from Swift/BAT, Swift/XRT, and MAXI. We also acquired contemporaneous low-resolution optical spectra with the 2.1 m OAN San Pedro Mártir (Mexico), the 2.1 m OAGH Cananea (Mexico), and the G.D. Cassini 1.5 m telescope in Loiano (Italy). The optical fractional root mean square is highest in the hard state and is best described by short-timescale flickering that is stronger at longer wavelengths. This suggests that the minute-timescale optical variability is driven by the jet in the hard state. In this scenario, the variability could be due to variations in the inflow that inject velocity fluctuations at the base of the jet (internal shock model). The variability is then quenched in the soft state, with any residual signal likely associated with variability in the accretion flow. This is in agreement with the accretion-ejection coupling in black hole binaries and confirms that the variability signature of the jet is present at optical wavelengths in all hard states. In the dimmest hard states, any residual optical variability may instead be linked to cyclo-synchrotron emission from the hot flow. The optical spectra reveal double-peaked emission lines and possible signatures of cold winds during the hard state. Such winds were previously reported in this source during the main 2018 outburst; here we provide one of the first tentative detections of their presence during the subsequent re-brightenings. The absence of optical wind signatures in the soft state likely reflects a higher level of disc ionisation driven by the increased X-ray flux, which suppresses the low ionisation features detectable in the optical band.