The Equilibrium Photoionized Absorber in 3C351

We present two ROSAT PSPC observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an ‘ionized absorber’ in its X-ray spectrum. The factor 1.7 change in flux in the ∼2 years between the observations allows a test of models for this ionized absorber. The absorption feature at ∼ 0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity and hence the ionizing flux at the absorber is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agrement quantitatively. The maximum response time of 2 years allows us to limit the gas density: ne > 2 × 10 4 cm; and the distance of the ionized gas from the central source R < 19 pc. This produces a strong test for a photoionized absorber in 3C 351: a factor 2 flux change in ∼1 week in this source must show non-equilibrium effects in the ionized absorber.