F-GAMMA: On the phenomenological classification of continuum radio spectra variability patterns of Fermi blazars

TL;DR

This study classifies the radio spectral variability patterns of Fermi blazars into five types, linking them to underlying physical mechanisms, and provides a framework for understanding their spectral evolution.

Contribution

It introduces a phenomenological classification of blazar radio spectra variability into five types, based on multi-frequency monitoring data and physical modeling.

Findings

78 sources classified into 5 variability types

Spectral evolution driven by shock-in-jet mechanisms

Achromatic variability possibly caused by Doppler boosting

Abstract

The F-GAMMA program is a coordinated effort to investigate the physics of Active Galactic Nuclei (AGNs) via multi-frequency monitoring of Fermi blazars. In the current study we show and discuss the evolution of broad-band radio spectra, which are measured at ten frequencies between 2.64 and 142 GHz using the Effelsberg 100-m and the IRAM 30-m telescopes. It is shown that any of the 78 sources studied can be classified in terms of their variability characteristics in merely 5 types of variability. It is argued that these can be attributed to only two classes of variability mechanisms. The first four types are dominated by spectral evolution and can be described by a simple two-component system composed of: (a) a steep quiescent spectral component from a large scale jet and (b) a time evolving flare component following the "Shock-in-Jet" evolutionary path. The fifth type is characterised by an achromatic change of the broad band spectrum, which could be attributed to a different mechanism, likely involving differential Doppler boosting caused by geometrical effects. Here we present the classification, the assumed physical scenario and the results of calculations that have been performed for the spectral evolution of flares.