Compton Dominance and the Blazar Sequence

TL;DR

This study investigates whether the blazar sequence is a real physical trend or a selection effect by analyzing Compton dominance and synchrotron peak frequencies in a Fermi LAT blazar sample, proposing a model explaining observed correlations.

Contribution

It introduces a simple model linking magnetic field, external radiation, and jet angle to explain blazar properties and the blazar sequence, including the absence of certain high-energy sources.

Findings

A correlation between Compton dominance and synchrotron peak frequency exists.

The model reproduces observed blazar trends and predicts future high-frequency blazars.

High synchrotron peak blazars with high Compton dominance are unlikely.

Abstract

Does the "blazar sequence" exist, or is it a result of a selection effect, due to the difficulty in measuring the redshifts of blazars with both high synchrotron peak frequencies (\gtrsim 10^{15} Hz) and luminosities (\gtrsim 10^{46} erg s^{-1})? We explore this question with a sample of blazars from the Second Catalog of Active Galactic Nuclei (AGN) from the Fermi Large Area Telescope (LAT). The Compton dominance, the ratio of the peak of the Compton to the synchrotron peak luminosities, is essentially a redshift-independent quantity, and thus crucial to answering this question. We find that a correlation exists between Compton dominance and the peak frequency of the synchrotron component for all blazars in the sample, including ones with unknown redshift. We then construct a simple model to explain the blazar properties in our sample, where the difference between sources is due to only the magnetic field of the blazar jet emitting region, the external radiation field energy density, and the jet angle to the line of sight, with the magnetic field strength and external energy density being correlated. This model can reproduce the trends of the blazars in the sample, and predicts blazars may be discovered in the future with high synchrotron peak frequencies and luminosities. At the same time the simple model reproduces the lack of high-synchrotron peaked blazars with high Compton dominances (\gtrsim 1).