Active Galactic Nuclei emit a substantial portion of their bolometric luminosities in X-rays. For example, the knots in radio jets are prominent sources of synchrotron X-rays while the hotspots of the brightest FRIIs emit self-synchrotron or Inverse Compton radiation. Most high-energy studies on flat-spectrum radio sources have been conducted for blazars which are dominant at $\gamma$-rays.

Augusto et al. (1998) have built a sample of 55 flat-spectrum radio sources dominated by structures (knots, hotspots, etc.) $\sim$0.1–2 kpc away from the nucleus. Seventeen (31%) of these are detected in X-rays (they tend to be the radio strongest) evenly splitting, morphologically, both at optical (radio) bands: nine QSO/BLLac (core-jets) on one-side; eight Galaxy/Sy2 (CSO/MSO/FRII) on the other. We have identified five confirmed compact/medium symmetric objects (CSO/MSOs) as X-ray emitters. A comparable type of source to CSO/MSOs is the physically similar (1–15 kpc) compact steep spectrum source (CSS), 28/129 (22%) of which are detected in X-rays, from a literature-selected sample (the percentage is smaller than for the 55-source sample due to a lower $<\!\!S_{4.85}\!\!>$). A 95% conf. level relation is found for CSSs: $S_X \propto (S_{4.85})^{0.6}$ and we found undistinguishable radio/X-ray properties for both the 55-source and CSS samples: clearly, their similar morphologies (e.g. knots in jets) stand up stronger than their radical radio spectrum differences.

Only two sources among the 55 (4%) have $\gamma$-ray detections and they seem quite abnormal (in $\alpha_{x\gamma}$ values, at least)–one of them is in a Sy2, not in a blazar.