Rotating jet phenomena in Active Galactic Nuclei

Abstract

Highly collimated and often bipolar outflows have been observed in many Active Galactic Nuclei (AGN). The present thesis considers in detail the implications of an intrinsic and an extrinsic caused rotation in such jets: The first part deals with the centrifugal acceleration of charged test particles at the base of an intrinsically rotating jet magnetosphere. The equation characterizing the radial accelerated motion is derived and an analytical solution presented. For electrons moving outwards along rotating magnetic field lines, the maximum attainable Lorentz factor is found to be limited to a few hundred by (i) inverse-Compton losses in the disk radiation field and (ii) the breakdown of the bead-on-the-wire approximation which occurs in the vicinity of the light cylinder. The results show that this mechanism may not only provide pre-accelerated seed particles required for efficient Fermi-type particle acceleration but could also contributes to the hard X-ray hump observed in AGN. The second part extends the investigation of the previous chapter. Following previous work by Webb (1989), steady state Green`s solutions of the kinetic equations describing the transport of energetic particles in a collisionless, rotating background flow are discussed for conditions assumed to prevail in the jets of AGN. By considering rigid and keplerian rotation profiles the centrifugal and shear acceleration of particles scattered by magnetic inhomogeneities are distinguished. In the case of rigidly rotating background flows shearing is absent and the energy gain is analogous to the bead-on-the-wire approach of part I. For keplerian rotation profiles both shear and centrifugal effects are present. In the case where the shear effects dominate, it is confirmed that power-law particle momentum solutions exist if the mean scattering time is an increasing function of momentum. The relevance of shear acceleration in addition to Fermi-type particle acceleration in AGN jets is finally pointed out with reference to recent observations (e.g. in 3C273). The third and final part considers the influence of an extrinsic caused jet rotation in the case of the well-kown AGN Mkn 501. It has been shown for the first time that the TeV and X-ray periodicity of ~23 days, observed in 1997, could be basically interpreted as a doppler-shifted flux modulation due to the orbital motion of the relativistic jet in a supermassive binary black hole system. Using typical jet parameters the intrinsic orbital period and the centre-of-mass distance have been determined, indicating a very close binary system. A simple upper limit on the allowed binary masses has been obtained assuming that the current binary separation is of such an order that gravitational radiation might become dominant, yielding black hole masses which interestingly agree with expectations from merger scenarios.