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 the 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.
Highlights
The Blazar Paradigm distributed under the terms andIn retrospect, we can identify the discovery of the first quasar, 3C 273 [1], as the discovery of the first blazar as well
The key features of blazar emission in different bands can be interpreted in light of the above-mentioned emission models, as follows: Radio: The radio emission of blazars is dominated by the beamed jet emission, and only at the sub-GHz frequency is there an emergence of radiation produced in the extended structures, such as hot spots and lobes
This becomes even more evident when five averaged broadband spectral energy distribution (SED) are built, adopting the radio luminosity as a bin criterion. This means that independently from the original classification of a blazar as Flat Spectrum Radio Quasars (FSRQs) or radio/X-ray-selected BL Lac, the average SEDs were built taking into account only the luminosity of the source at 5 GHz, available for all objects
Summary
We can identify the discovery of the first quasar, 3C 273 [1], as the discovery of the first blazar as well. Blazars come in two flavors: Flat Spectrum Radio Quasars (FSRQs) and BL Lac objects, depending on the width of their emission lines. The ADAF regime is radiatively inefficient and characterized by a dramatic decrease in ionizing UV photons that are unable to photo-ionize the clouds of gas responsible for the broad lines (e.g., see [11]) This coarse division into BL Lac objects and FSRQs, is in some cases arbitrary as transitional objects displaying mixed properties exist (e.g., see [12]). The SED of 3C 454.3 is interpreted as the superposition of the jet emission (two broad bumps peaking at optical and gamma-ray energies), the IR torus, and the hot corona.
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