 Inverse Compton scattering

Inverse Compton scattering transfers energy from an energetic electron to a photon in a Compton scattering event. Transforming into the electron's rest frame blueshifts an approaching photon; scattering there and transforming back can produce a much more energetic outgoing photon.

For a head-on collision with an ultrarelativistic electron of Lorentz factor $\gamma$, the low-recoil limit gives an approximately forward maximum photon energy $E_{\gamma ,\max }\simeq 4{\gamma }^2ϵ$, where $ϵ$ is the incident photon energy. This requires $\gamma ϵ\ll m_e{c}^2$; recoil limits the increase outside that regime.

A laboratory source overlaps a laser beam with an electron beam and selects scattered photons within an angular acceptance. Electron energy spread, beam divergence and collection angle affect the emitted bandwidth, so one photon-energy value does not characterize every collected event. ^[ref]