Fermi Blazars in the Zwicky Transient Facility Survey: Properties of Large Optical Variations

We analyze the optical light-curve data, obtained with the Zwicky Transient Facility (ZTF) survey, for 47 gamma-ray blazars monitored by the Large Area Telescope onboard {\it the Fermi Gamma-ray Space Telescope (Fermi)}. These 47 sources are selected because they are among the Fermi blazars with the largest optical variations in the ZTF data. Two color-magnitude variation patterns are seen in them, one being redder to stable when brighter (RSWB; in 31 sources) and the other being stable when brighter (in 16 sources). The patterns fit with the results recently reported in several similar studies with different data. Moreover, we find that the colors in the stable state of the sources share similar values, which (after corrected for the Galactic extinction) of most sources are in a range of 0.4--0.55. This feature could be intrinsic and may be applied in, for example, the study of intragalactic medium. We also determine the turning points for the sources showing the RSWB pattern, after which the color changes saturate and become stable. We find a correlation between optical fluxes and gamma-ray fluxes at the turning points. The physical implications of the correlation remain to be investigated, probably better with a sample of high-quality gamma-ray flux measurements.