Radio/gamma-ray time delay in the parsec-scale cores of active galactic nuclei

TL;DR

This study detects a consistent time delay with gamma-ray emission leading radio in active galactic nuclei, suggesting gamma rays originate near the radio core, with delays mainly due to synchrotron opacity effects.

Contribution

It provides the first large-scale analysis of radio/gamma-ray time delays in AGNs, revealing that gamma-ray emission precedes radio emission by about 1.2 months in the source frame.

Findings

Gamma-ray emission leads radio emission by 1-8 months.

Delay peaks at approximately 1.2 months in the source frame.

Synchrotron opacity is the primary cause of the observed delay.

Abstract

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.