Similarity of the Optical-IR and Gamma-Ray Time Variability of Fermi
  Blazars
  We present the time variability properties of a sample of six blazars, AO
0235+164, 3C 273, 3C 279, PKS 1510-089, PKS 2155-304, and 3C 454.3, at
optical-IR as well as gamma-ray energies. These observations were carried out
as a part of the Yale/SMARTS program during 2008-2010 that has followed the
variations in emission of the bright Fermi-LAT-monitored blazars in the
southern sky with closely-spaced observations at BVRJK bands. We find the
optical/IR time variability properties of these blazars to be remarkably
similar to those at the gamma-ray energies. The power spectral density (PSD)
functions of the R-band variability of all six blazars are fit well by simple
power-law functions with negative slope such that there is higher amplitude
variability on longer timescales. No clear break is identified in the PSD of
any of the sources. The average slope of the PSD of R-band variability of these
blazars is similar to what was found by the Fermi team for the gamma-ray
variability of a larger sample of bright blazars. This is consistent with
leptonic models where the optical-IR and gamma-ray emission is generated by the
same population of electrons through synchrotron and inverse-Compton processes,
respectively. The prominent flares present in the optical-IR as well as the
gamma-ray light curves of these blazars are predominantly symmetric, i.e., have
similar rise and decay timescales, indicating that the long-term variability is
dominated by the crossing time of radiation or a disturbance through the
emission region rather than by the acceleration or energy-loss timescales of
the radiating electrons. In the blazar 3C 454.3, which has the highest-quality
light curves, the location of a large gamma-ray outburst during 2009 December
is consistent with being in the jet at ~18 pc from the central engine. This
poses strong constraints on the models of high energy emission in the jets of
blazars.
