A relativistic model of the radio jets in 3C296

We present new, deep 8.5-GHz VLA observations of the nearby, low-luminosity radio galaxy 3C 296 at resolutions from 0.25 to 5.5 arcsec. These show the intensity and polarization structures of the twin radio jets in detail. We derive the spectralindex distribution using lower-frequency VLA observations and show that the flatterspectrum jets are surrounded by a sheath of steeper-spectrum diffuse emission. We also show images of Faraday rotation measure and depolarization and derive the apparent magnetic-field structure. We apply our intrinsically symmetrical, decelerating relativistic jet model to the new observations. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the jets are inclined by 58◦ to the line of sight. Their outer isophotes flare to a half-opening angle of 26◦ and then recollimate to form a conical flow beyond 16 kpc from the nucleus. On-axis, they decelerate from a (poorly-constrained) initial velocity β = v/c ≈ 0.8 to β ≈ 0.4 around 5 kpc from the nucleus, the velocity thereafter remaining constant. The speed at the edge of the jet is low everywhere. The longitudinal profile of proper emissivity has three principal power-law sections: an inner region (0 – 1.8 kpc), where the jets are faint, a bright region (1.8 – 8.9 kpc) and an outer region with a flatter slope. The emission is centre-brightened. Our observations rule out a globally-ordered, helical magnetic-field configuration. Instead, we model the field as random on small scales but anisotropic, with toroidal and longitudinal components only. The ratio of longitudinal to toroidal field falls with distance along the jet, qualitatively but not quantitatively as expected from flux freezing, so that the field is predominantly toroidal far from the nucleus. The toroidal component is relatively stronger at the edges of the jet. A simple adiabatic model fits the emissivity evolution only in the outer region after the jets have decelerated and recollimated; closer to the nucleus, it predicts far too steep an emissivity decline with distance. We also interpret the morphological differences between brightness enhancements (“arcs”) in the main and counter-jets as an effect of relativistic aberration.

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