Radiation force on relativistic jets in active galactic nuclei

Abstract

Radiative deceleration of relativistic jets in active galactic nuclei as the result of inverse Compton scattering of soft photons from accretion discs is discussed. The Klein-Nishina (KN) cross-section is used in the calculation of the radiation force resulting from inverse Compton scattering. Our result shows that deceleration owing to scattering in the KN regime is important only for jets starting with a bulk Lorentz factor larger than 103. When the bulk Lorentz factor satisfies this condition, particles scattering in the Thomson regime contribute positively to the radiation force (acceleration), but those particles scattering in the KN regime are dominant and the overall effect is deceleration. In the KN limit, the drag resulting from Compton scattering, although less severe than in the Thomson limit, strongly constrains the bulk Lorentz factor. Most of the power from the deceleration goes into radiation and hence the ability of the jet to transport significant power (in particle kinetic energy) out of the subparsec region is severely limited. The deceleration efficiency decreases significantly if the jet contains protons and the proton to electron number density ratio satisfies the condition np/ne0 > 2γmin/μp where γmin is the minimum Lorentz factor of relativistic electrons (or positrons) in the jet frame and μp is the proton to electron mass ratio.