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Astron. Astrophys. 353, 473-478 (2000)

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1. Introduction

The origin of the nonthermal, highly variable emission in active galactic nuclei (AGN) has been widely discussed. Several acceleration mechanisms have been proposed which may explain the observed high energy emission extending up to TeV energies at least in three blazars (Mkn 421, Mkn 501, 1ES 2344+514: e.g. Catanese 1999). Fermi-type particle acceleration mechanisms in relativistic jets seem to be very effective but require a seed population of electrons with Lorentz factors of at least 100. Up to now, it remains a problem to be solved, how this pre-acceleration is achieved (e.g. Kirk et al. 1994).

Since the pioneering work of Gold in the late 1960s (Gold 19681969), centrifugal driven outflow of matter has often been discussed in the context of pulsar emission theory (for recent contribution see e.g. Machabeli & Rogava 1994; Chedia et al. 1996; Gangadhara 1996; Contopoulos et al. 1999). In the case of accreting black hole systems (e.g. AGN) Blandford and Payne (1982) first pointed out that centrifugal driven outflows (jets) from accretion disks are possible, if the poloidal field direction is inclined at an angle less than [FORMULA] to the radial direction. In such models, a rotating magnetosphere could emerge from an accretion disk or the rotating black hole itself (Blandford & Znajek 1977) initiating a plasma outflow with initially spherical shape until the flow is collimated on a scale of less than a few hundred Schwarzschild radii (e.g. Camenzind 19951996; Fendt 1997). For a rapidly rotating black hole system, the critical angle mentioned above could be as large as [FORMULA] (Cao 1997).

In magnetohydrodynamical scenarios for the origin of relativistic jets, centrifugal acceleration is rather limited, leading to maximum bulk Lorentz factors of the order of 10 (Camenzind 1989). Nevertheless, it seems quite interesting whether supra-thermal test particles (e.g. from magnetic flares on the accretion disk) could be accelerated to even higher energies by such rotating magnetospheres. Recently, Gangadhara & Lesch (1997) proposed a model for spinning active galactic nuclei in which charged test particles are accelerated to very high energies by the centrifugal force while moving along rotating magnetic field lines. According to their calculations, the nonthermal X-ray and [FORMULA]-ray emission in AGN could arise via the inverse-Compton scattering of UV-photons by centrifugal accelerated electrons.

In this paper, we reinvestigate the acceleration of charged test particles in an idealized two-dimensional model where the magnetic field rotates rigidly with a fraction of the rotational velocity of the black hole (cf. Fendt 1997). Centrifugal acceleration occurs as a consequence of the bead-on-the-wire motion. A charged particle gains rotational energy as long as it is directed outwards but we show that its energy gain is substantially limited not only by inverse-Compton losses but also by the effects of the relativistic Coriolis force.

Based on an analysis of forces, the special relativistic equation of motion is derived in Sect. 2. This equation is solved in closed form in Sect. 3. Sect. 4 gives an estimate for the maximum Lorentz factor attainable in the case of AGN. The results are discussed in the context of the particle acceleration problem for rotating AGN jets in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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