BL Lac Objects (2) ...
It has been found that some AGNs, often in the blazar class, emit extremely high energy
photons.
Gamma Ray Flares
For example, the adjacent false-color image obtained by the EGRET gamma ray
telescope aboard the Compton Gamma Ray Observatory
shows the blazar/quasar 3C279 emitting a flood of
gamma rays in 1991 (the color coding represents gamma ray intensity, with white the largest).
This is termed a gamma ray flare. For a while this flare
made 3C279 one of the brightest objects in the gamma ray sky, before it faded from view at
gamma ray energies.
High-Energy Photons
In addition to sudden flares
in intensity at gamma ray wavelengths, some AGNs have been
observed to emit photons of incredible energy. For example, gamma rays having energy of
order a trillion electron-Volts (a hundred billion times the energy
required to ionize a hydrogen atom)
have been observed from Markarian 421,
a B-Lac lying at a redshift of z = 0.031. Gamma rays, in particular gamma rays of
this energy, cannot be produced by normal thermal processes for any temperature that we can
reasonably expect in an AGN.
Compton Scattering
The production of high-energy photons in AGNs is thought to involve a process called
inverse Compton scattering.
The above left figure illustrates. In
Compton scattering, illustrated in the top part of
the figure, a high-energy photon strikes an electron and accelerates it, giving up energy in
the process.
(Compton scattering is named for Arthur Holly Compton, an American physicist who shared the 1927
Nobel Prize for his discovery that photons could scatter electrons. This discovery
was important in establishing the reality of photons and the correctness of quantum mechanics.
The Compton Gamma Ray Observatory
was also named in his honor.)
Inverse Compton scattering is then just the reverse of Compton scattering, as illustrated in
the bottom part of the figure: a high-energy electron strikes a photon, imparting energy
and shortening the wavelength of the photon.
