Connection between optical and gamma-ray variability in blazars
  We use optical data from the Palomar Transient Factory (PTF) and the Catalina
Real-Time Transient Survey (CRTS) to study the variability of gamma-ray
detected and non-detected objects in a large population of active galactic
nuclei (AGN) selected from the Candidate Gamma-Ray Blazar Survey and Fermi
Gamma-Ray Space Telescope catalogs. Our samples include 714 sources with PTF
data and 1244 sources with CRTS data. We calculate the intrinsic modulation
index to quantify the optical variability amplitude in these samples. We find
the gamma-ray detected objects to be more variable than the non-detected ones.
The flat spectrum radio quasars (FSRQs) are more variable than the BL Lac
objects in our sample, but the significance of the difference depends on the
sample used. When dividing the objects based on their synchrotron peak
frequency, we find the low synchrotron peaked (LSP) objects to be significantly
more variable than the high synchrotron peaked (HSP) ones, explaining the
difference between the FSRQs and BL Lacs. This could be due to the LSPs being
observed near their electron energy peak, while in the HSPs the emission is
caused by lower energy electrons, which cool more slowly. We also find a
significant correlation between the optical and gamma-ray fluxes that is
stronger in the HSP BL Lacs than in the FSRQs. The FSRQs in our sample are also
more Compton dominated than the HSP BL Lacs. These findings are consistent with
models where the gamma-ray emission of HSP objects is produced by the
synchrotron self-Compton mechanism, while the LSP objects need an additional
external Compton component that increases the scatter in the flux-flux
correlation.
