5 Observational relevance

The present results may serve as an instructive example revealing the significant potential of shear and centrifugal acceleration in the jets of AGN. Since intrinsic jet rotation is typically expected in the AGN setting, the proposed mechanism might operate in a natural manner over a wide range. This suggests that particle acceleration in a rotating and shearing background flow could be of particular relevance for an explanation of the continuous optical emission observed from several AGN jets (e.g. for 3C273; M87; PKS 0521-365; cf. Meisenheimer et al. 1997; Jester et al. 2001, 2002), as in contrast to shock acceleration, this mechanism generally would not be constrained to localized regions. Recent observations in fact indicate that the radiating particles might be widely distributed so that the optical emission from radio jets for example, is not confined to bright individual knots (cf. Meisenheimer et al. 1996, 1997). In order to explain the apparent absence of strong radiative cooling between knots, Meisenheimer et al. (1997) suggested the operation of an extended so-called "jet-like'' acceleration mechanism, which was thought to be associated with velocity shear and expected to contribute in addition to standard diffusive shock acceleration. Meanwhile, the need of such an extended acceleration mechanism is strengthened by recent, high-resolution HST observations of the jet in the 3C273 (Jester et al. 2001, 2002). Classified as a blazar source, 3C273 is known as the brightest and nearest (z=0.158) quasar, with a jet extending up to several tens of kpc. The absence of a detectable counter-jet suggests bulk relativistic motion even up to kpc-scales, a conclusion also supported by recent models for the X-ray emission observed with Chandra (cf. Sambruna et al. 2001). The observations performed by Jester and collaborators have revealed an extremely well-collimated jet, where the optical spectral index varies only smoothly along the jet indicating only smooth variations of physical conditions. Their phenomenological analysis yields an optical spectral flux index of $\sim$-(0.65-0.8) and provides strong evidence for an underlying, universal powerlaw electron distribution, which is maintained almost entirely throughout the jet. If in the context of shear and centrifugal acceleration, the particle energy is locally dissipated by synchrotron radiation, the spectral emissivity $j_{\rm\nu}$ for a power-law particle number density distribution $n \propto p'^2~f \propto p'^{-\delta}$ is given by $j_{\rm\nu}\propto \nu^{-s}$ with $s=(\delta-1)/2$. Thus, for s=(0.65-0.8) we require $\delta=(2.3-2.6)$, which for example, may be realized for the case of a constant diffusion coefficient and high flat rotation or for the case of a simple shear flow with moderate momentum-dependent diffusion.