The equilibrium photoionized absorber in 3C 351

We present two ROSAT Position Sensitive Proportional Counter (PSPC) observa tions of the radio-loud, lobe-dominated quasar 3C 351, which shows an "ioni zed absorber" in its X-ray spectrum. The factor of 1.7 change in flux in th e similar to 2 years between the observations allows a test of models for t his ionized absorber. The absorption feature at similar to 0.7 keV (quasar frame) is present in b oth spectra but with a lower optical depth when the source intensity-and he nce the ionizing flux at the absorber-is higher, in accordance with a simpl e, single-zone, equilibrium photoionization model. Detailed modeling confir ms this agreement quantitativeiy. The maximum response time of 2 years allo ws us to limit the gas density: n(e) > 2 x 10(4) cm(-3); and the distance R of the ionized gas from the central source is less than 19 pc. This produc es a strong test for a photoionized absorber in 3C 351: a factor of 2 flux change in similar to 1 week in this source must show nonequilibrium effects in the ionized absorber.