The relationship between the X-ray and radio components in the compact steep-spectrum quasar 3C 48

We combine results from ROSAT, Chandra and multifrequency VLBA observations of the compact steep-spectrum quasar 3C 48 in an attempt to understand why the radio source is so small and unusual. The X-ray spectrum shows no evidence for the excess absorption which might have allowed us to conclude that 3C 48 is small because it is bottled up by cold neutral gas. We infer that the X-ray spectrum of the nucleus is made up of a soft, variable, steep-spectrum component, and harder, power-law emission of slope consistent with the 1-GHz radio spectrum. The similarity of the X-ray-to-radio ratio of 3C 48 to that seen in core-dominated radio-loud quasars leads us to examine the possibility that the harder X-ray emission is inverse Compton radiation from the radio source, which is more than 99 per cent resolved in our VLBA data. The weak (3sigma) evidence that we find for a proper motion of 0.5+/-0.2c in a compact radio component about 0.05 arcsec from the core implies that if this component has a highly relativistic bulk motion, it is at a very small angle to the line of sight. However, stringent requirements on the jet opening angle make it unlikely that all the X-ray emission is from a fast jet which sees boosted cosmic microwave background emission and emits beamed X-rays in the observer's frame. If the unusual radio structures are intrinsically one-sided and unbeamed, the inverse Compton mechanism can provide an appreciable fraction of the X-ray emission if the magnetic field strength is a factor of 6-10 below that which gives equal energy in radiating relativistic particles and magnetic fields and roughly minimizes the total energy in the source. It remains possible that the unresolved X-ray emission arises from close to the central engine, either as an embedded blazar or associated with the accretion processes.