BL Lac Objects (3) ...

A specific way in which an AGN could produce a high-energy photon by non-thermal processes is illustrated in the adjacent right figure. The AGN generates copious amounts of light at UV energies and below by thermal emission from the accretion disk and secondary heating of the surrounding material. As we have seen, the UV from the disk can photoionize clouds in the broad line region and these clouds then emit light largely at optical wavelengths.

If some of this lower frequency light enters the relativistic jet, it can undergo an inverse Compton boost to higher frequency. We illustrate in the figure for an optical photon that inverse Compton scatters from an electron in the jet. For relativistic jets, this boost can be very large. For example, if the velocity of an electron in the jet is v = 0.9999995c, the frequency is increased by a factor of a million through inverse Compton scattering (see the box below). This is sufficient to raise a microwave photon to X-ray energies, an optical photon to intermediate-energy gamma rays, and an X-ray photon to trillion electron-Volt gamma ray energies.

Some Inverse Compton
Boost Factors
v / c Boost Factor
0.0 1.00
0.5 1.33
0.9 5.26
0.995 102
0.99995 104
0.9999995 106
Technically Speaking: Inverse Compton Boosts

For highly relativistic electrons, the increase in frequency for a photon by the inverse Compton process is given by a rather simple formula. If the initial frequency of the photon is f0,

f / f0 = 1 / [1 - (v / c)2]

where f is the final (boosted) frequency and the quantity on the right side of the equation is called the boost factor, since multiplying the original frequency by it gives the final frequency. The table gives the boost factor for several velocities. Notice that the velocity must be large relative to the speed of light (relativistic) to give large boosts in frequency by the inverse Compton process. For non-relativistic velocities the boost factor is close to one (no increase in frequency). Let's use the table to do an example. Suppose that the electron velocity is v = 0.9999995c. Then the boost factor is a million and

f = f0 x Boost Factor = 5 x 1014 Hz x 106 = 5 x 1020 Hz

for an optical photon of frequency 5 x 1014 Hz. The boosted photon now has a frequency in the gamma ray region of the spectrum.