Testing of Inverse Compton Models for Active Galactic Nuclei with Gamma‐Ray and Radio Observations
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Abstract
We compare the Metsähovi 22 and 37 GHz total flux density variation data of active galactic nuclei with EGRET (on board the Compton Gamma Ray Observatory) gamma-ray observations from Cycles 1-4. Statistical analysis reveals that there is a genuine connection between the high-frequency radio properties of the sources, e.g., the type of the source, the phase of the radio flare, the associated radio variability brightness temperature, and the gamma-ray emission. Typically the most likely gamma-ray emitter is a high optical polarization quasar with a large Doppler boosting factor. The highest levels of gamma-ray emission are observed during the initial (or peak) stages of high-frequency radio flares. BL Lac objects are much weaker gamma-ray emitters, with no clear correlation between radio and gamma variations. We calculate the distance of the gamma-ray-emitting region from the core in quasars and obtain an average value of 4.9 pc, well downstream from the accretion disk or the broad-line region. This indicates that the strongest time-dependent gamma-ray emission of quasars originates in the relativistic jet, within the same shocks that produce the synchrotron flares at radio frequencies induced by the synchrotron self-Compton mechanism. Weaker gamma-ray emission does not seem to be associated with shocks and may be caused by other mechanisms, such as external Compton processes.