Nonlinear radiative cooling of relativistic particles under equipartition conditions

I. Instantaneous monoenergetic injection

Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany e-mail: rsch@tp4.ruhr-uni-bochum.de

Received: 14 June 2007
Accepted: 4 September 2007

Abstract

Context.In powerful cosmic nonthermal radiation sources with dominant magnetic-field self generation, the generation of magnetic fields at almost equipartition strength by relativistic plasma instabilities operates as fast as the acceleration or injection of ultra-high energy radiating electrons and hadrons in these sources. Consequently, the magnetic field strength becomes time-dependent and adjusts itself to the actual kinetic energy density of the radiating electrons in these sources. This coupling of the magnetic field and the magnetic field energy density to the kinetic energy of the radiating particles changes both the synchrotron emissivity and the intrinsic temporal evolution of the relativistic particle energy spectrum after injection.

Aims.The nonlinear kinetic equation for the intrinsic temperoral evolution of relativistic electrons is solved for the case of instantaneous injection of monoenergetic particles.

Methods.Analytical derivations and graphical illustrations.

Results.In blazar and gamma-ray burst sources, the nonlinear synchrotron cooling of each particle under equipartition conditions is then orders of magnitude quicker than the linear cooling behaviour in constant magnetic-field strength sources. This dramatic reduction for the intrinsic radiation loss time may be essential for understanding the observed rapid time variation, of the order of days in the case of the non-blazar radio galaxy M 87 and minutes in flaring blazar jets such as PKS 2155-304. Significant differences in the optically-thin, synchrotron spectral distributions at different times and in the synchrotron light curves at different frequencies are predicted.