Astron. Astrophys. 349, 685-690 (1999)

1. Introduction

Many extragalactic jets originating in active galactic nuclei appear in radio maps as bright sources of radiation that is believed to result from synchrotron emission by relativistic electrons. In some cases continuous radiation extends to optical and even to X-ray frequencies, implying the presence of highly relativistic electrons with TeV energies (Eilek & Hughes 1991). Some of the radio sources have severe lifetime problems, however, in that the time for the electrons to be carried out to the lobes (even with a relativistic jet speed) is longer than their radiation lifetime. Hence local reacceleration of the radiating particles is necessary in the jet.

Direct electric field in the jet can accelerate electrons out of a thermal background very efficiently. Jets appear to be strongly magnetized plasma flows, and the dissipation of magnetic field by virtue of magnetic reconnection is believed to explain the observed source luminosity (Romanova & Lovelace 1992; Blackman 1996). Magnetic energy is assumed to be released in the reconnecting current sheet, or possibly multiple sheets, formed in the jet when plasma motions create magnetic field lines that are not coaligned. Reconnection is accompanied by a DC electric field in the sheet, which acts to accelerate charged particles. Direct collisions are very infrequent in the low-density plasmas of radio jets. Therefore, unless anomalous resistivity effects lead to much more efficient scattering, reconnection is collisionless and a large portion of the magnetic field energy is converted to the particle kinetic energy. Observations of radio galaxies indicate that an acceleration mechanism, associated with velocity shear, is required in addition to the usual shock-front acceleration mechanism (Meisenheimer et al. 1997). Because the shear should create magnetic field configurations with nonparallel field lines that eventually reconnect, reconnection is likely to be this additional acceleration mechanism.

Two distinct DC field acceleraton models can be considered. One model assumes that the particle motion inside the sheet is one-dimensional-along the electric field. Effects of the magnetic field are ignored inside the sheet. Acceleration lengths in this model are large because they are limited by the synchrotron energy losses only (or by the Coulomb losses in denser plasmas). Hence the electric field has to be correspondingly weak in order to avoid a tremendous potential drop that the particles would experience otherwise. The other model explores the effects of the magnetic field on the particle motion in the current sheet. A nonzero magnetic field inside the sheet leads to the Lorentz force that ejects the particles across the sheet. Thus smaller acceleration lengths and stronger electric fields are predicted. Both the weak and the strong electric field models were suggested, for example, in solar flare physics in order to explain the acceleration of hard X-ray generating electrons in flares (see Miller et al. 1997for a review).

The weak field model has already been applied to the electron acceleration in extragalactic radio jets (Lesch & Birk 1998). It is the goal of this paper, therefore, to present the strong field model and point out its advantages. Observationally, the magnetic field in extragalactic jets appears to possess both toroidal and axial components, justifying this intention (Thomson et al. 1993). Furthermore, the low electric field strength in the weak field approach implies a slow reconnection rate as measured by the reconnection Alfvén Mach number . It is not known what regime would better describe magnetic reconnection in extragalactic jets, but fast reconnection models with have been quite successful in interpreting charged particle acceleration in the geomagnetic tail (Speiser 1965) and on the Sun (Martens 1988; Litvinenko 1996). Hense this paper investigates the strong electric field, fast reconnection regime.

The paper is organized as follows. Sect. 2 briefly reviews the weak field model and shows that it corresponds to slow reconnection in the current sheet (). Pursuing an alternative approach, Sect. 3 discusses charged particle orbits in the current sheet in the fast reconnection regime. Sect. 4 demonstrates that the strong field approach leads to reasonable estimates for the electron energies and acceleration times in the extragalactic jets. The findings are summarized in Sect. 5.

© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999
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