Online Luminosity Monitor at Belle II

We describe a system used for online measurements of luminosity, utilizing elastic $e^+e^-$ Bhabha scattering and two-photon annihilation processes reconstructed with the Belle II electromagnetic calorimeter. The Belle II experiment at the SuperKEKB asymmetric-energy $e^+e^-$ collider is designed to achieve a luminosity of $6.3\times10^{35}$ $\text{cm}^{-2}\text{s}^{-1}$. With the designed parameters of SuperKEKB, the statistical accuracy of the instantaneous luminosity measurement provided by the Online Luminosity Monitor is expected to be better than $1\%$ within one second. The overall systematic uncertainty is estimated to be at the level of $1\%$. Comparison with a dedicated offline analysis and results on the long-term stability of the monitor's performance are also presented.