On the relation between AGN gamma-ray emission and parsec-scale radio jets

TL;DR

This study finds a strong correlation between gamma-ray emission and parsec-scale radio jet brightness in AGN, indicating that brighter, Doppler-boosted jets are more likely to emit gamma rays, with flares occurring within months of each other.

Contribution

It demonstrates a clear link between radio and gamma-ray activity in AGN jets, highlighting the parsec-scale core as a common origin for flares and emphasizing the role of Doppler boosting.

Findings

Gamma-ray flux correlates with compact radio flux density.

LAT-detected jets have higher brightness temperatures.

Radio activity increases within months of gamma-ray detection.

Abstract

We have compared the radio emission from a sample of parsec-scale AGN jets as measured by the VLBA at 15 GHz, with their associated gamma-ray properties that are reported in the Fermi LAT 3-month bright source list. We find in our radio-selected sample that the gamma-ray photon flux correlates well with the quasi-simultaneously measured compact radio flux density. The LAT-detected jets in our radio-selected complete sample generally have higher compact radio flux densities, and their parsec-scale cores are brighter (i.e., have higher brightness temperature) than the jets in the LAT non-detected objects. This suggests that the jets of bright gamma-ray AGN have preferentially higher Doppler-boosting factors. In addition, AGN jets tend to be found in a more active radio state within several months from LAT-detection of their strong gamma-ray emission. This result becomes more pronounced for confirmed gamma-ray flaring sources. We identify the parsec-scale radio core as a likely location for both the gamma-ray and radio flares, which appear within typical timescales of up to a few months of each other.