A small fraction (~10%) of galaxies in our universe has a compact, highly luminous (L~10⁴⁴⁻⁴⁸ ergs/s) central region that emits radiation comparable to or sometimes even higher than the radiation emitted by the rest of the galaxy. Such galaxies often show strong broad emission line spectra and produce variable emissions that extend from radio to X-rays or in some cases up to very high energy γ−rays. These galaxies are called ‘active galaxies’ and their nuclei are termed as ‘Active Galactic Nuclei’ (AGNs). Some AGNs are known to host relativistic bipolar outflows or jets that emit non-thermal continuum emission from radio to γ−rays. Because of these extraordinary features, AGNs have been attracting researchers for a long time.

In 1908, E.A. Fath at Lick Observatory detected strong emission lines in the optical spectra of nebula NGC 1068 which was different from the usually seen absorption spectra of normal galaxies. Later, Carl Seyfert, in 1943, also observed strong broad emission lines in the optical spectra of six spiral galaxies having high central surface brightness and hence realized that they form a distinct class (Seyfert, 1943).
In the late 1950s, the first radio surveys of the sky were performed and several catalogues e.g. the Third Cambridge Catalogue (3C), were published. Hazard et al.(1963) investigated the spectrum of the radio source 3C 273 by the lunar occultation method and observed broad emission lines in it. Later, Schmidt (1963) studied the optical spectra of 3C 273 and obtained a redshift of z = 0.158, identifying it as an extragalactic source. He also realized that the star-like radio source 3C 273 was actually the nuclear region of a galaxy that was about 100 times brighter than the galaxy at optical wavelengths. Such radio sources formed a new class of AGNs and were named quasars (or QSOs; Quasi-Stellar Objects) by Hong-Yee Chiu, in 1964.
The ultimate energy source and the radiation mechanisms of AGNs were a big mystery. The idea of accretion of matter onto a supermassive black hole (SMBH) was proposed in the 1960s to explain this phenomenon by several astrophysicist (e.g. Salpeter, 1964; Lynden-Bell, 1969). The discovery of rapid X-ray flare from Cygnus X-1, a Galactic X-ray source, by Oda et al. (1971) triggered the possibility for the presence of an accreting black hole in AGNs. Later, Rees (1984) suggested that the source of radiation from the AGN was an accreting SMBH lying at the center of the AGN.

According to the current paradigm, the fundamental components of AGN, Figure 1.1, are followings (Ghisellini, 2013):

• Supermassive Black Hole (SMBH):

A black hole with mass M_BH~10^6-9 M_Sun at the center, which pulls matter from the surroundings due to its high gravitational field.

• Accretion Disk (AD):

Matter attracted by the gravity of the central SMBH forms a spiraling disk, known as ‘accretion disk’.

• X-ray corona:

The AD is sandwiched by a corona of hot plasma which inverse Comptonizes the optical/ultraviolet (UV) disk photons to produce X-rays.

• Broad Line Region (BLR):

A region of small rapidly (~3000 km/s) moving, dense (~10¹⁰cm⁻³) gas clouds is located at a distance of ~0.1−1.0 pc from the SMBH. A fraction (10%) of the disk radiation is reprocessed by these clouds to give rise to emission lines in the spectra. These emission lines get broaden due to Doppler shifts and hence this region is known as ‘broad line region’.

• Obscuring Torus:

At a distance of several parsecs (~1-10 pc) from the SMBH, there exists a dusty torus which obscures the radiations from the central regions (SMBH, AD, and BLR) and re-emits them in the infrared (IR) bands.

• Narrow Line Region (NLR):

Similar to BLR, there is another region of relatively less dense (~10³cm⁻³) gas clouds at larger distance (~100 pc) from the SMBH. These clouds are moving with lower velocities (~300 km/s) and are responsible for the narrow lines in the spectra. This region is known as ‘narrow line region’.

• Relativistic Jets:

Jets are the magnetically collimated bipolar outflows of relativistic plasma and are often observed in certain AGNs. They are thought to originate from the vicinity of SMBH and can extend up to few kpc (or sometimes up to Mpc) distances.