Nonthermal radiation from jets of active galactic nuclei: Electrostatic bremsstrahlung as alternative to synchrotron radiation

Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

Corresponding author: rsch@tp4.ruhr-uni-bochum.de

Received: 1 April 2003
Accepted: 4 July 2003

Abstract

The interaction of collimated relativistic hadronic and leptonic jet outflows from active galactic nuclei with the surrounding interstellar and intergalactic medium generates by two-stream instabilities huge intensities of subluminal electrostatic (Langmuir) wave fields in the co-moving outflow region with energy densities larger than the energy density in the magnetic field. We therefore revisit electrostatic bremsstrahlung, i.e. the inverse Compton scattering of electrostatic waves by energetic positrons and electrons to photons (), as an alternative nonthermal radiation process to synchrotron radiation which in the literature sofar has been the standard low-frequency radiation process for radiation. It is known that this radiation process has similar polarisation properties as synchrotron radiation. By calculating Doppler boosted luminosity spectra in case of leptonic outflows we demonstrate that in jet outflow sources electrostatic bremsstrahlung by swept-up relativistic electrons is stronger than synchrotron radiation from the same electrons in the radio to optical frequency band, whereas the optically thick bulk electrostatic bremsstrahlung from the non-relativistic pair outflow plasma is typically a factor 10¹⁰ smaller. Our analysis is based on photon angle integrated monochromatic approximations of the electrostatic bremsstrahlung power and synchrotron radiation power in a plasma, which are constructed from moments of the respective radiation powers of single electrons and positrons. Specialising to the specific subluminal electrostatic wave distribution and the swept-up relaivistic electrons in the leptonic jet model of Schlickeiser et al. ([CITE]) we calculate the expected Doppler boosted radiation intensities from bulk electrostatic bremsstrahlung, relativistic electron bremsstrahlung and synchrotron radiation for standard inner jet parameters. This requires to investigate in detail (i) the equilibrium distribution of the radiating electrons and (ii) the emission and absorption coefficients of each radiation process. An attractive feature of electrostatic bremsstrahlung mechanism is the fact that the interaction of the jet outflow with the surrounding ambient medium generates both the target electrostatic plasma waves and the radiating swept-up relativistic electrons. It does not rely on the existence of large intrinsic ordered magnetic fields to account for strong emission due to the synchrotron radiation process. This might be of importance also for nonthermal emission far away from the central jet engine.