Collective Evidence for Inverse Compton Emission from External Photons in High-Power Blazars

TL;DR

This study provides collective evidence that high-power blazar jets are dominated by inverse Compton emission from external photons, with the dominance increasing as the jet orientation aligns more closely with our line of sight, especially in the most powerful sources.

Contribution

It demonstrates that external Compton emission dominates in high-power jets and establishes a correlation between jet power, orientation, and emission mechanisms, which was not clearly shown before.

Findings

High-power jets show increased Compton dominance with decreasing orientation angle.

External Compton emission is favored over synchrotron self-Compton in powerful jets.

No correlation between Compton dominance and orientation in low-power jets.

Abstract

We present the first collective evidence that Fermi-detected jets of high kinetic power (L_kin) are dominated by inverse Compton emission from upscattered external photons. Using a sample with a broad range in orientation angle, including radio galaxies and blazars, we find that very high power sources (L_kin > 10^45.5 erg s^{-1}) show a significant increase in the ratio of inverse Compton to synchrotron power (Compton dominance) with decreasing orientation angle, as measured by the radio core dominance and confirmed by the distribution of superluminal speeds. This increase is consistent with beaming expectations for external Compton (EC) emission, but not for synchrotron self Compton (SSC) emission. For the lowest power jets (L_kin < 10^43.5 erg s^{-1}), no trend between Compton and radio core dominance is found, consistent with SSC. Importantly, the EC trend is not seen for moderately high power flat spectrum radio quasars with strong external photon fields. Coupled with the evidence that jet power is linked to the jet speed (Kharb et al. 2010), this finding suggests that external photon fields become the dominant source of seed photons in the jet comoving frame only for the faster and therefore more powerful jets.