Broad-band Jet Emission in Young and Powerful Radio Sources: the Case of the CSS Quasar 3C 186

TL;DR

This study investigates the X-ray emission of the CSS quasar 3C 186, exploring jet contributions and proposing a structured jet model with a blazar component to explain the observed X-ray flux.

Contribution

It introduces a structured jet model with a blazar component to account for X-ray emission in CSS quasars, challenging simpler single-zone leptonic scenarios.

Findings

Single-zone leptonic models cannot fully explain the X-ray emission.

A structured jet with a blazar component can account for the observed X-ray flux.

Jet deceleration within the host galaxy influences X-ray emission and jet-medium interactions.

Abstract

We present the X-ray analysis of a deep ~200 ksec Chandra observation of the compact steep spectrum radio-loud quasar 3C 186 (z=1.06) and investigate the contribution of the unresolved radio jet to the total X-ray emission. The spectral analysis is not conclusive on the origin of the bulk of the X-ray emission. In order to examine the jet contribution to the X-ray flux, we model the quasar spectral energy distribution (SED), adopting several scenarios for the jet emission. For the values of the main physical parameters favored by the observables, a dominant role of the jet emission in the X-ray band is ruled out when a single zone (leptonic) scenario is adopted, even including the contribution of the external photon fields as seed photons for inverse Compton emission. We then consider a structured jet, with the blazar component that- although not directly visible in the X-ray band - provides an intense field of seed synchrotron photons Compton-scattered by electrons in a mildly relativistic knot. In this case the whole X-ray emission can be accounted for if we assume a blazar luminosity within the range observed from flat spectrum radio quasars. The X-ray radiative efficiency of such (structured) jet is intimately related to the presence of a complex velocity structure. The jet emission can provide a significant contribution in X-rays if it decelerates within the host galaxy, on kiloparsec scales. We discuss the implications of this model in terms of jet dynamics and interaction with the ambient medium.