The Blazar sequence and its Physical Understanding

TL;DR

This paper reviews the development and current understanding of the blazar sequence, highlighting recent observational advances, especially in gamma-ray and TeV bands, and discusses its physical basis and criticisms.

Contribution

It provides an updated version of the blazar sequence incorporating TeV-detected objects and discusses the physical interpretation and debates surrounding the sequence.

Findings

Updated blazar sequence including TeV-detected objects

Identification of MeV and hard-TeV blazars as key classes

Discussion of physical models and criticisms

Abstract

Introduced in 1998 to attempt a first unified view of the broad-band emission properties of blazars, the blazar sequence has been extensively used in the past 25 years to guide observations as well as physical interpretation of the overall emission from these galaxies. In this review, we describe the evolution of the sequence along with the tremendous advances in the observational field, in particular in the gamma-ray band. A new version of the sequence built on TeV-detected objects is also presented. Two extreme classes of objects (MeV and hard-TeV blazars) are included in the discussion, given their relevance for future observatories. Finally, the current physical understanding at the base of the sequence is presented along with the major criticisms to the blazar sequence.