  We analyzed optical/X-ray quasi-simultaneous light curves of Aql X-1,
obtained by MAXI (Monitor of All-sky X-ray Image), ZTF (Zwicky Transient
Facility) and LCO (Las Cumbres Observatory) in about 3.6 years from 2016, for
understanding electromagnetic radiation mechanisms during its outbursts. As a
result, we confirmed that 5 outbursts had detected in the epoch, and that 3
outbursts underwent the X-ray state transition across Low-Hard, In-Transition,
and High-Soft state while remaining 2 outbursts stayed in the Low-Hard state.
We found that the optical spectral energy distribution in the High-Soft state
is consistent with a simplified irradiated disk model, and that the optical
color/magnitude variation can be explained by variations in the X-ray
luminosity and the disk geometrical thickness.
