Radiative Processes In Extragalactic Large-Scale Jets

Abstract

Abstract. About one thousand extragalactic large-scale jets are known, and a few tens of them are confirmed sources of infrared, optical, or X-ray photons. Multiwavelength emission comming directly from these outflows is always non-thermal in origin. This fact constitutes a primary difficulty in extracting unknown parameters of large-scale jets, since the non-thermal featureless continua do not allow to infer undoubtfully (or even at all) bulk velocities and composition of the radiating plasma. In addition, arcsecond spatial resolution, limited sensitivity and narrow energy bands of the best high-frequency telescopes like Spitzer, Hubble and Chandra, preclude precise constraints on the spectral and morphological properties of the discussed objects. Nevertheless, new multiwavelength observations have substantially enriched our knowledge on extragalactic large-scale jets, in many aspects, however, by means of challenging previous predictions and expectations. In this short contribution I will concentrate on the following issue: what can be learned by analyzing broad-band emission of the discussed objects about particle acceleration processes acting thereby and about jet internal parameters.Keywords: acceleration of particles -radiation mechanisms: non-thermal -galaxies: jets PARTICLE ACCELERATION PROCESSESPolarization and power-law spectral distribution of radio emission observed from extragalactic large-scale jets imply its synchrotron origin, and thus a power-law energy distribution of the radiating ultrarelativistic electrons. Radio spectral indices are concentrated around α R ∼ 0.75 (see, e.g., Kataoka & Stawarz 2005). This value is not strictly predicted by any particular model of particle acceleration applied to the discussed objects. For example, assuming the 'universal' shock-like particle spectrum n e (γ) ∝ γ −2 usually considered in this context, where γ is the particle Lorentz factor, one expects the synchrotron continuum of the form S ν ∝ ν −α with the spectral index α = 0.5 in a weak cooling regime, and α = 1.0 in a strong synchrotron (expected to be dominant) cooling regime. The situation is therefore that even the observed radio spectral properties of the large-scale jet are not consistent with the simplest scenarios for the evolution of the electron energy distribution. Taking into account the complexity of the jet phenomenon we should not in fact expect the simplest models to be realistic. Meanwhile, the 'injection' electron spectrum (not affected by spectral ageing) remains elusive. One can hope that by performing observations at low radio frequencies such a spectrum will be revealed, and thus strong observational constraints will help to develop the appropriate theoretical model. Indeed, Young et al. (2005) found that the radio data for FR I sources imply n e (γ) ∝ γ −2.1 . Such a spectrum may be interpreted as manifestation of the effects con-