Blazars in the Fermi Era: The OVRO 40-m Telescope Monitoring Program
  The Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope
provides an unprecedented opportunity to study gamma-ray blazars. To capitalize
on this opportunity, beginning in late 2007, about a year before the start of
LAT science operations, we began a large-scale, fast-cadence 15 GHz radio
monitoring program with the 40-m telescope at the Owens Valley Radio
Observatory (OVRO). This program began with the 1158 northern (declination>-20
deg) sources from the Candidate Gamma-ray Blazar Survey (CGRaBS) and now
encompasses over 1500 sources, each observed twice per week with a ~4 mJy
(minimum) and 3% (typical) uncertainty. Here, we describe this monitoring
program and our methods, and present radio light curves from the first two
years (2008 and 2009). As a first application, we combine these data with a
novel measure of light curve variability amplitude, the intrinsic modulation
index, through a likelihood analysis to examine the variability properties of
subpopulations of our sample. We demonstrate that, with high significance
(7-sigma), gamma-ray-loud blazars detected by the LAT during its first 11
months of operation vary with about a factor of two greater amplitude than do
the gamma-ray-quiet blazars in our sample. We also find a significant (3-sigma)
difference between variability amplitude in BL Lacertae objects and
flat-spectrum radio quasars (FSRQs), with the former exhibiting larger
variability amplitudes. Finally, low-redshift (z<1) FSRQs are found to vary
more strongly than high-redshift FSRQs, with 3-sigma significance. These
findings represent an important step toward understanding why some blazars emit
gamma-rays while others, with apparently similar properties, remain silent.
