[Paper Review] Compton Dominance and the Blazar Sequence

Justin D. Finke|Dec 4, 2012

Astrophysics and Cosmic Phenomena1 references5 citations

TL;DR

This paper investigates whether the blazar sequence—correlations between synchrotron peak frequency and luminosity—is a physical phenomenon or a selection effect. Using Fermi-LAT blazar data, it shows that Compton dominance (ratio of Compton to synchrotron peak luminosities) correlates with synchrotron peak frequency across all blazars, including those without redshifts. The authors propose a simple model where jet magnetic field, external radiation field, and viewing angle explain the observed trends, and predict that high-synchrotron-peaked, luminous blazars will be discovered in the future, consistent with observations and ruling out models with uncorrelated parameters.

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).

Motivation & Objective

• To determine whether the blazar sequence is a physical correlation or a selection effect due to redshift measurement difficulties.
• To use Compton dominance—a redshift-independent quantity—as a diagnostic tool to probe the blazar sequence without relying on redshifts.
• To test whether the observed correlations in the synchrotron peak frequency and luminosity are consistent with a physical model of jet emission.
• To assess whether alternative models, such as uncorrelated parameters in Monte Carlo simulations, can reproduce the observed data.
• To predict the future discovery of high-synchrotron-peaked, luminous blazars based on the proposed model.

Proposed method

• The study uses a sample of blazars from the Fermi-LAT Second Catalog of AGN, focusing on those with measured or estimated Compton dominance.
• Compton dominance is calculated as the ratio of peak Compton luminosity to peak synchrotron luminosity, a redshift-independent metric.
• A simple physical model is constructed assuming that differences among blazars arise from variations in jet magnetic field strength, external radiation field energy density, and viewing angle.
• The model assumes a correlation between magnetic field strength and external radiation field energy density to reproduce observed trends.
• The model is tested against observed Compton dominance–synchrotron peak frequency correlations and compared with alternative models, such as those with uncorrelated parameters.
• The model predicts that high-synchrotron-peaked, high-luminosity blazars will be found in future surveys, particularly those with highly aligned jets.

Experimental results

Research questions

• RQ1 Is the blazar sequence a physical correlation or a selection effect due to redshift measurement bias?
• RQ2 Does Compton dominance correlate with synchrotron peak frequency across all blazars, including those without known redshifts?
• RQ3 Can a simple model based on jet magnetic field, external radiation field, and viewing angle reproduce the observed Compton dominance–synchrotron peak frequency correlation?
• RQ4 Do models assuming uncorrelated parameters (e.g., Giommi et al. 2012a) predict the absence of high-Compton-dominance, high-synchrotron-peaked blazars, as observed?
• RQ5 What are the observational signatures that could distinguish between models where the Compton emission arises from external Compton scattering versus synchrotron self-Compton processes?

Key findings

• A significant correlation exists between Compton dominance and synchrotron peak frequency across all blazars in the Fermi-LAT sample, including those with unknown redshifts.
• The observed lack of high-Compton-dominance, high-synchrotron-peaked blazars (i.e., no sources with $A_C \gtrsim 1$ and $\nu^{sy}_{pk} \gtrsim 10^{15}$ Hz) rules out models with uncorrelated parameters, such as those in Giommi et al. (2012a).
• The proposed model, which correlates magnetic field strength and external radiation field energy density, successfully reproduces the observed trends in Compton dominance and synchrotron peak frequency.
• The model predicts that future surveys will discover high-synchrotron-peaked, luminous blazars, particularly those with jets highly aligned with the line of sight.
• The model is consistent with the absence of bright, high-peaked blazars in current samples, suggesting that such sources are not missing due to selection effects but are intrinsically rare or require specific conditions.
• The results support the physical reality of the blazar sequence, with Compton dominance serving as a robust, redshift-independent diagnostic for understanding jet physics and accretion modes.