inverse-Compton scattering
/ KOMP-ton /
Picture a slow-moving ball drifting along, when a fast tennis racket smacks into it. The ball flies off much faster than before — it stole energy from the racket. Inverse-Compton scattering is that collision, but between a low-energy photon (a particle of light) and a super-fast electron. The electron is the racket; the photon is the ball. When they collide, the photon bounces away carrying far more energy than it came in with, while the electron loses a little of its speed.
The ordinary Compton effect, discovered in 1923, is the reverse: a high-energy photon hits a slow electron and gives the electron a kick, so the photon comes out with less energy. Astrophysics often runs this backwards. Space is full of gentle photons — the cosmic microwave background, starlight, infrared dust glow — and it is also full of electrons whipped to near light-speed by shocks and magnetic fields. When a fast electron meets a soft photon, it can boost that photon's energy enormously, turning a radio or visible photon into an X-ray or gamma ray. Roughly, the photon's energy is multiplied by a factor that grows with the square of how close the electron is to light-speed, so a single scatter can lift a photon by a factor of millions.
Inverse-Compton scattering is a key way the universe makes its highest-energy light. It powers much of the gamma-ray glow of blazars, helps cool the hot electrons around black holes, and limits how bright a synchrotron source can get before its own emitted light starts getting scattered up to even higher energies. It pairs naturally with synchrotron radiation: the same fast electrons that make synchrotron light by spiraling in magnetic fields also up-scatter ambient photons by inverse-Compton, and disentangling the two is a central puzzle in modeling high-energy sources.
In a blazar's jet, electrons moving at nearly light-speed plow through a sea of low-energy photons. Each collision can lift a soft photon all the way to gamma-ray energies — so the same jet that glows in radio by synchrotron also blazes in gamma rays by inverse-Compton, from the very same electrons.
A fast electron kicks a soft photon up to X-ray or gamma-ray energy — Compton run in reverse.
It is 'inverse' only relative to ordinary Compton scattering: here the electron gives energy to the photon, not the other way around. The total energy is conserved — the photon's gain is the electron's loss — so it is a way the universe gradually cools its fast electrons.