Optical and ultraviolet monitoring of the black hole X-ray binary MAXI J1820+070/ASASSN-18ey for 18 months

MAXI J1820+070 is a low-mass black hole X-ray binary system with high luminosity in both optical and bands during the outburst periods. We present extensive photometry X-ray, ultraviolet, bands, as well densely-cadenced spectra, covering phase from beginning of to $\sim$550 days. During rebrightening process, emission preceded by 20.80 $\pm$ 2.85 The spectra are characterized blue continua features Balmer series, He I, II lines broad Bowen blend. pseudo equivalent width (pEW) found show anticorrelations flux measured at comparable phases, which due increased suppression continuum. At around peak, full half maximums (FWHMs) H$_{\beta}$ $\lambda$4686 tend stabilize 19.4 Angstrom 21.8 Angstrom, corresponds line forming region radius 1.7 1.3 R_sun within disk. further analyzed absolute fluxes that positive correlations flux, favoring irradiation model responsible for emission. However, fact experiences dramatic drop t$\sim$200 days after outburst, while only shows little variations suggests additional energy such viscous may contribute radiation addition irradiation.