INVERSE COMPTON X-RAY-EMISSION FROM THE SUPERLUMINAL QUASAR 3C 345

In quasars with strong radio cores, the inverse-Compton process is bel ieved to be the dominant source X-ray emission. For objects with parse c-scale radio jets, simple models have predicted that components in th e jet emerging from the quasar nucleus generate the observed X-ray emi ssion. We have tested this hypothesis in detail for the quasar 3C 345 using a ROSAT X-ray observation in 1990 July, together with quasi-simu ltaneous VLBI imaging of the parsec-scale jet at five frequencies. The ROSAT spectrum is well fitted by a power law with index alpha = -0.96 +/- 0.13, consistent with models in which the X-ray emission results from inverse-Compton scattering of radio radiation from high-energy el ectrons in compact components. We show that the radio properties of br ightest '' knot '' in the jet ('' C5 '') can be fitted with a homogene ous sphere model whose parameters require bulk relativistic motion of the emitting material; otherwise the predicted inverse-Compton X-ray e mission exceeds the observed flux. If C5 is the origin of the X-ray em ission, then it has a Doppler factor delta = 7.5(-2)+3. If the nucleus or other components contribute to the X-ray emission, then this becom es a firm lower limit to delta. The inhomogeneous jet model of Konigl is a good fit both to the barely resolved (< 1 pc) flat-spectrum nucle us in the radio, and also to the ROSAT X-ray spectrum. The synchrotron and inverse-Compton emitting fluid moves down a narrow cone (opening angle 2phi almost-equal-to 5-degrees) nucleus relativistically, with d elta almost-equal-to 4.6. Doppler factors for the nucleus and C5, deri ved from our ROSAT observation, provide evidence for bulk relativistic motion in the jet. By combining these constraints with the well-known superluminal motion of jet components, we can deduce the jet geometry . For epoch 1990.5 we infer the Lorentz factor gamma = 7.5(-1.5)=1.0 a nd angle to the line of sight theta = 8(-3-degrees)+2-degrees for H-0 = 100 km s-1 Mpc-1. These values are the most reliable yet derived usi ng this method, because of the near-simultaneity of our X-ray and VLBI observations and the quality of the multifrequency VLBI images and co mponent radio spectra.