Gamma-ray Selected AGN: What We See, What We Miss, and Why It Matters

Those AGNs who detected by gamma ray are called Gamma ray selected AGN.

Gamma rays are the most energetic form of light produced in extreme cosmic conditions. Gamma ray is the highest energy range of photons. 

Not all AGNs are the same. Most AGNs such as Seyfert and radio quiet AGNs shine brightly in X-rays or IR, but they are invisible in gamma rays. AGNs that have jets in an up and down direction and one of these jets is almost directly towards the Earth, that AGN are called Blazars. These jets are almost super bright and emit non-thermal radiation. and this radiation is Doppler boosted.

• Non-Thermal: their energy is so much not due to heat but due to accelerated particles.
• Blazars produce the most gamma rays, so such blazars are called champions of gamma ray production. 
• Blazars can also emit neutrinos and cosmic rays, making them multi-messenger sources.

SED (Spectral Energy Distribution) of Blazars: 

• Graph of energy and frequency coming from Blazer: show two humps. This double peak is called SED.

• First hump: Synchrotron radiation (electrons + magnetic field). this peak range from IR to X-rays.

• Second hump: Inverse Compton scattering ya Hadronic processes. this peak range from X-ray to gamma rays.

• But every blazer has a different SED - some have a strong 1st hump, some have a strong 2nd.

Synchrotron radiation: synchrotron radiation happens when fast-moving charge particle like electron a spiral around magnetic field these particles are relativistically, meaning they move close to the speed of light.

Inverse Compton scattering: As charged particles move through the accretion disc, they produce light that appears to us as bright flashes in UV and optical light. When photons from this produced light travel up (or down) from the disc and collide with energetic electrons there, they convert the UV light into X-rays, a process known as inverse Compton scattering.

Source: https://www.researchgate.net/figure/Definition-of-different-blazar-types-based-on-the-peak-of-the-synchrotron-component-n-S_fig1_258258440

On the basis of SEB, Blazars are 3 types: LSP, ISP, and HSP.

1. Low synchroton peaked (LSP): Mostly found in FSRQs. Peak frequency is less than 10^14 Hz.
2. Intermediate Synchroton Peaked (ISP): Mixed found in FSRQs and BL Lacs. Peak frequency is 10^ 14 to 10^15.
3. High Synchroton Peaked (HSP): Mostly found in BL Lacs. peak frequency is more than 10^15 Hz.

Types of Blazars:

1. BL Lacs: less luminous, less Bright disk, less redshift means nearby + low luminous, little/no emission line and jets dominating, BH mass around 10^8 times of sun's mass,
2. FSRQs (Flat Spectrum radio Quasars): more luminous (around 10^46 erg/sec), more Bright disk, more redshifts means more powerful and ancient, strong emission lines, BH mass around 10^9 times of sun's mass.

Q.1 Which AGN produces gamma-rays?

Ans: Not all AGN produce gamma-rays. Blazars (main source of gamma-rays), Radio Galaxies (some nearby ones) and Rarely: Starburst + AGN mix. 

Q.2 How are gamma-rays produced in AGN?

Ans: Inverse Compton, Synchrotron, etc.

Q.3 Where are gamma-rays produced in AGN?

Ans: Inside relativistic jet.

Q.4 What do gamma-ray bursts tell us about AGN?

Ans: Gamma-rays give us important clue into the internal physics of AGN, Gamma-ray Background (EBG), Power aur Energetics ofAGN, Nature Radiation, jet orientation, energy output, and even cosmic evolution and many other things.

We detect different gamma-ray energy ranges with two types of detectors: 

1. High Energy (HE): 100 MeV – 100 GeV
2. Very High Energy (VHE): 50 GeV – 10 TeV.

• Most gamma ray AGN are BL Lacks AGN, especially in VHE band.
• Blazers don't just glow, they glow, flare and explode for periods ranging from a few seconds to a few hours. "That's because the jets are unstable.
• Blazer shoots a Gamma Ray that hit a detector. So remember a blazer is a galaxy with a jet pointing to us this jets powered by black hole shining across all light.

The majority of sources in the Fermi-LAT (Large area telescope) catalogue are blazers. Fermi-LAT is a space telescope that observes gamma-ray bursts (from 100 MeV to 300 GeV).

Different wavelength bands detect different blazars:

Band	Mainly Detects
Radio/Microwave	Mostly FSRQ / LSP blazars (low synchrotron peak)
X-ray	Mostly BL Lac / HSP blazars (high synchrotron peak)
Gamma-ray	Sensitive to HSP BL Lacs (kyunki unka gamma-ray spectrum hard hota hai)

Meaning, from which band you observe, that band will decide what you will be able to detect.

Selection effect: If we are able to detect only some specific things in an observation or survey, and other similar things are missed - simply because they are outside the limits of the instrument, method, or closed choice - then that effect is called the selection effect. for ex. If our camera detects only red colour, then you will see only red flowers – but this does not mean that there are only red flowers in the world.

If blazars winks at camera. a checklist appers:

• jet pointed at us
• synchroton+ gamma ray emssion
• rapid variability
• detected across the EM spectrum