THE ROSAT SPECTRUM OF 3C-351 - A WARM ABSORBER IN AN X-RAY-QUIET QUASAR

3C 351 is one of the most X-ray-quiet radio quasars (alpha(OX) approxi mately 1.6). We have observed 3C 351 with the ROSAT position sensitive proportional counter (PSPC) and find a complex X-ray spectrum which i s not well reproduced by a power law plus low-energy cutoff model. Sof t excess, partial covering, and ''warm absorber'' models can all produ ce acceptable fits, although only the warm absorber model gives typica l values for the high-energy continuum slope. The alpha(OX) measured b y using quasi-simultaneous ROSAT, MMT, and HST observations is in the range 1.5-1.6, significantly above the average of 1.37 (sigma approxim ately 0.15) for a complete sample of 33 3CR quasars. If the soft exces s or partial covering models are correct, 3C 351 appears X-ray-quiet i n the PSPC band because it has an extremely steep or flat intrinsic hi gh-energy spectral slope. However, if the warm absorber model is corre ct, the quasar is intrinsically X-ray-quiet; the normalization of the intrinsic (unabsorbed) X-ray emission is unusually low relative to the optical luminosity. We investigate the properties of our warm absorbe r model in some detail. The apparently complicated behavior of the fit parameters may be understood by considering the effects of changing a bsorbing column and ionization parameter on intrinsic power-law spectr a of different slopes.