The Blazar sequence and its Physical Understanding

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.

arXiv comment18 pages, 9 figures, 2 tables, invited review in Galaxies special issue on "Extragalactic TeV Astronomy"

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• The Physical Properties of Changing-look Blazars
• The Fermi-LAT view of the changing-look blazar OQ 334
• Hadronic signatures from magnetically dominated baryon-loaded AGN jets
• Classification of the \emph{Fermi}-LAT Blazar Candidates of Uncertain type using eXtreme Gradient Boosting
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