Description
We report the detection of a non-zero time delay between radio emission measured by the VLBA at 15.4GHz 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. For the correlation analysis, we used 0.1-100GeV {gamma}-ray photon fluxes, taken from monthly binned measurements from the first Fermi LAT catalog (1FGL; Abdo et al. 2010, Cat. J/ApJS/188/405), and 15.4GHz radio flux densities from the MOJAVE VLBA program (Lister et al. 2009, Cat. J/AJ/137/3718). 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 15GHz 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 approximately 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.
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