On Multiwavelength Emission and Morphology of Large-Scale Quasar Jets

We propose, that the knots of the large-scale jets in powerful radio sources represent moving and separate portions of the jet matter, with the excess kinetic power. This can explain many morphological properties of quasar jets — like high knot-tointerknot brightness contrasts, frequency-independent knot profiles and almost universal extents of the knot regions — independently of the exact emission mechanisms responsible for producing broad-band jet emission. We briefly discuss a possible connection between this scenario and the idea of highly modulated/intermittent activity of the jet engine. Growing numbers of large-scale jets in powerful extragalactic radio sources are now being observed at optical and X-ray frequencies (for a recent review see [8]). The high angular resolution of Hubble Space Telescope and Chandra X-ray Observatory coupled with VLA results have given us the unique opportunity to analyze also the morphological characteristics of jets at different photon energies. The implied properties, like (i) knotty morphology with a high knotto-interknot brightness contrast, (ii) frequency-independent knots’ profiles and similar extents of the knots when observed at different photon energies (∼ 1 kpc), (iii) lack of correlation between spectral and brightness changes along the jet, and (iv) spatial offsets between the maxima of the X-ray and radio emission within the knot regions, have to be explained by any model regarding multiwavelength emission of the large-scale quasar jets. All these characteristics are difficult to understand if the knots are identified with strong stationary (e.g., reconfinement) shocks through which the jet matter flows continuously. As discussed in [10], other possibilities (like the ‘clumping model’, [12]) are also in many aspects problematic. In Stawarz et al. ([10]) we proposed instead, that the knots represent portions of a relativisticaly moving jet with excess kinetic power. Such a situation is expected if the activity of central engines in radio-loud AGNs is intermittent, or highly modulated. Then, many morphological properties of the large-scale quasar jets can be explained in a natural way, almost independently of the exact emission processes involved in producing the jet broad-band spectra. The idea of intermittent jet activity was considered previously in the context of ‘partial’ radio jets associated with the radio galaxies 3C 219 and 3C 288 [2,3],