Upper panel: γ-ray light curve of CTA 102 with 1-day time bins from 01 January 2016 to 01 April 2018. Lower panels: γ-ray spectra in the energy range from 100 MeV to 300 GeV for the periods that showed significant deviation from the simple power-law model. The power-law with exponential cut-off spectral model (dashed red line) with the fit uncertainties (red solid lines) are shown together with the spectral points and it is compared with other adopted models (broken power-law in blue and log parabola in black). The spectral points are obtained by separately running gtlike for smaller energy intervals.