The Equilibrium Photoionized Absorber in 3C 351

We present two ROSAT Position Sensitive Proportional Counter (PSPC) observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an "ionized absorber" in its X-ray spectrum. The factor of 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 agreement quantitatively. The maximum response time of 2 years allows us to limit the gas density: ne>2×104 cm-3; and the distance R of the ionized gas from the central source is less than 19 pc. This produces a strong test for a photoionized absorber in 3C 351: a factor of 2 flux change in ~1 week in this source must show nonequilibrium effects in the ionized absorber.