HST optical spectral index map of the jet of 3C 273

Abstract

Abstract. We present HST images at 622 nm and 300 nm of the jet in 3C 273 and determine the run of the optical spectral index at 0. 2 along the jet. The smoothness of spectral index changes shows that the physical conditions are varying smoothly across the jet. There is no correlation between the optical flux and spectral index, as would be expected for relativistic electrons suffering strong cooling due to synchrotron emission. We find no evidence for localized acceleration or loss sites. This suggests that the spectral shape is not changing much throughout the jet. We show that relativistic beaming and/or sub-equipartition magnetic fields cannot remove the discrepancy between light-travel time along the jet and the lifetime of electrons emitting optical synchrotron radiation. We consider this further evidence in favour of a distributed electron acceleration process.

X-rays from the jet in 3C 273: Clues from the radio–optical spectra

“…A similar flattening was already seen between α UV opt determined with the HST and the optical spectral index determined on ground-based data at slightly lower frequencies (Fig. 4 in Jester et al 2001).…”

“…The WFPC2 data have been described in Jester et al (2001). Details of the NICMOS data are contained in Jester (2001) and will be reported elsewhere (Jester et al, in prep.…”

Particle acceleration in rotating and shearing jets from AGN

“…The formation of power law particle spectra under a wide range of conditions reveals the significant potential of shear and centrifugal acceleration as a natural explanation for the origin of the extended, continuous emission recently observed from AGN (e.g. Meisenheimer et al 1997;Jester et al 2001). In the present paper we aim to provide a more detailed investigation of this model, including the derivation of the relevant particle transport equation (cf.…”

“…for 3C273; M87; PKS 0521-365; cf. Meisenheimer et al 1997;Jester et al 2001Jester et al , 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.…”

Fermi acceleration in astrophysical jets

“…Stochastic acceleration like shear or second-order emission up to ν obs ∼ 5 · 10 17 b 2 0 Hz < ∼ 2 b 2 0 keV. Neglecting inverse Compton losses for a moment, it seems possible that synchrotron emission from electrons accelerated via second-order Fermi processes can, at least in principle, account for the observed extended emission in the optical (Jester et al, 2001), and perhaps even in the Chandra X-ray regime (cf. Harris and Krawczynski, 2006).…”