2021MNRAS.504.4226S

MAXI J1820+070 is a low-mass black hole X-ray binary system with high luminosity in both optical and X-ray bands during its outburst periods. We present extensive photometry in the X-ray, ultraviolet, and optical bands, as well as densely cadenced optical spectra, covering the phase from the beginning of the optical outburst to ∼550 days. During the rebrightening process, the optical emission precedes the X-ray by 20.80 ± 2.85 days. The spectra are characterized by blue continua and emission features of Balmer series, He I, and He II lines and a broad Bowen blend. The pseudo equivalent widths (pEWs) of emission lines are found to show anticorrelations with the X-ray flux measured at comparable phases, due to increased suppression by the optical continuum. At around the X-ray peak, the full widths at half-maximum (FWHMs) of Hβ and He IIλ4686 tend to stabilize at 19.4 and 21.8 Å, corresponding to the line-forming regions at radii of 1.7 and 1.3 R_☉ within the disc. We further analysed the absolute fluxes of the lines and found that the fluxes of Hβ and He IIλ4686 show positive correlations with the X-ray flux, favouring the suggestion that the irradiation model is responsible for the optical emission. However, the fact that X-ray emission experiences a dramatic flux drop at t ∼ 200 days after the outburst, while the optical flux only shows a little variation, suggests that additional energy such as viscous energy may contribute to the optical radiation, in addition to X-ray irradiation.