Blazars are a peculiar class of active galactic nuclei (AGNs) that have their relativistic jets pointed close to the line of sight to the observer with angle ≤ 10° (Antonucci 1993; Urry & Padovani 1995). They are classified as flat-spectrum radio quasars (FSRQs) and BL Lacerate (BL Lac) objects based on the strength of the emission lines in their optical/infrared (IR) spectrum. Both classes of objects emit radiation over the entire accessible electromagnetic spectrum from low-energy radio to high-energy γ-rays. As blazars are aligned close to the observer, the emission is highly Doppler boosted, causing them to appear as bright sources in the extragalactic sky. They dominate the extragalactic γ-ray sky first hinted by the Energetic Gammma-ray Experiment Telescope (EGRET) observations onboard the Compton Gamma-Ray Observatory (CGRO; Hartman et al. 1999) and now made apparent by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Atwood et al. 2009). The broad-band spectra of blazars are dominated by emission from the jet with weak or absent emission lines from the broad-line region (BLR). One of the defining characteristics of blazars is that they show flux variations (Wagner & Witzel 1995) over a wide range of wavelengths on time-scales ranging from months to days and minutes. In addition to flux variations they also show large optical and radio polarization as well as optical polarization variability. In the radio band they have flat spectra with the radio spectral index (αr)  0.5 ($S_{\nu } \propto \nu ^{-\alpha _r})$. The broad-band spectral energy distribution (SED) of blazars is characterized by a two-hump structure, one peaking at low energies in the optical/IR/X-ray region and the other one peaking at high energies in the X-ray/MeV region (Fossati et al. 1998; Mao et al. 2016). In the one-zone leptonic emission models, the low-energy hump is due to synchrotron emission processes and the high-energy hump is due to inverse Compton (IC) emission processes (Abdo et al. 2010b). The seed photons for the IC process can be either internal to the jet (synchrotron self-Compton or SSC; Konigl 1981; Marscher & Gear 1985; Ghisellini & Maraschi 1989) or external to the jet (external Compton or EC; Begelman et al. 1987). In the case of EC, the seed photons can be from the disc (Dermer & Schlickeiser 1993; Boettcher, Mause & Schlickeiser 1997), the BLR (Sikora, Begelman & Rees 1994; Ghisellini & Madau 1996), and the torus (Błaerrorzdotejowski et al. 2000; Ghisellini & Tavecchio 2008). Though leptonic models are found to fit the observed SED of a majority of blazars, for some blazars, their SEDs are also well fitted by either hadronic (Mücke et al. 2003; Böttcher et al. 2013) or lepto-hadronic models (Diltz & Böttcher 2016; Paliya et al. 2016). In the hadronic scenario, the γ-ray emission is due to synchrotron radiation from extremely relativisitic protons (Mücke et al. 2003) or the cascade process resulting from proton–proton or proton–photon interactions (Mannheim 1993). Even during different brightness/flaring states of a source, a single emission model is not able to fit the broad-band SED at all times. For example in the source 3C 279, while the flare during 2014 March–April is well fitted by a leptonic model (Paliya, Sahayanathan & Stalin 2015b), the flare in 2013 December with a hard γ-ray spectrum is well described by lepto-hadronic processes (Paliya et al. 2016). Thus, the recent availability of multiwavelength data coupled with studies of sources at different active states indicates that we still do not have a clear understanding of the physical processes happening close to the central regions of blazars.
Citation: Fossati et al. 1998
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