Correlations between Optical/Infrared and Gamma-ray Variability for Bright Blazars Monitored in 2008-2017

We report the results of cross correlations of the SMARTS optical/infrared and Fermi-LAT gamma-ray light curves for 8 bright blazars that have been monitored with 1 day resolution over the past decade. For the temporal correlation analysis of unevenly sampled variability data, we use the Discrete Correlation Function (DCF), creating an empirical bootstrapping method to assess the significance of the DCF amplitude for each blazar. Our results are perhaps surprising. Early on in the Fermi mission, the brightest gamma-ray blazar 3C 454.3 showed zero lag between optical/infrared and gamma-ray fluxes as reported by Bonning et al. (2012), which was consistent with the leptonic model that optical/infrared photons are produced by synchrotron radiation of relativistic electrons and gamma rays are produced by inverse Compton scattering of ambient photons by the synchrotron-emitting electrons. However, among the 8 blazars, only one blazar - 3C 454.3 - shows a significant peak at zero lag, and the other 7 blazars show no significant peak at zero lag. Some blazars show broad peaks at tens of days of lags at or just below 3 sigma significance. In addition, for a given blazar, strong changes of the DCFs from one epoch to the next are shown by the analyses of time periods of one or two year. These results make it complicated to understand blazar emission mechanisms. We discuss possible physical explanations.