Radio/gamma-ray time delay in the parsec-scale cores of active galactic
  nuclei
  We report the detection of a non-zero time delay between radio emission
measured by the VLBA at 15.4 GHz and gamma-ray radiation (gamma-ray leads
radio) registered by the Large Area Telescope (LAT) on board the Fermi
Gamma-Ray Space Telescope for a sample of 183 radio and gamma-ray bright active
galactic nuclei (AGNs). For the correlation analysis we used 100 MeV - 100 GeV
gamma-ray photon fluxes, taken from monthly binned measurements from the first
Fermi LAT catalog, and 15.4 GHz radio flux densities from the MOJAVE VLBA
program. The correlation is most pronounced if the core flux density is used,
strongly indicating that the gamma-ray emission is generated within the compact
region of the 15 GHz VLBA core. Determining the Pearson's r and Kendall's tau
correlation coefficients for different time lags, we find that for the majority
of sources the radio/gamma-ray delay ranges from 1 to 8 months in the
observer's frame and peaks at about 1.2 months in the source's frame. We
interpret the primary source of the time delay to be synchrotron opacity in the
nuclear region.
