The Equilibrium Photoionized Absorber in 3C 351

We present two ROSAT PSPC observations of the radio-loud, lobe-dominatedquasar 3C 351, which shows an `ionized absorber' in its X-ray spectrum. Thefactor 1.7 change in flux in the $\sim$2~years between the observations allowsa test of models for this ionized absorber. The absorption feature at ~0.7 keV (quasar frame) is present in both spectrabut with a lower optical depth when the source intensity - and hence theionizing flux at the absorber - is higher, in accordance with a simple,single-zone, equilibrium photoionization model. Detailed modeling confirms thisagrement quantitatively. The maximum response time of 2 years allows us tolimit the gas density: n_e > 2 x 10^4 cm^{-3}; and the distance of the ionizedgas from the central source R < 19 pc. This produces a strong test for aphotoionized absorber in 3C~351: a factor 2 flux change in ~1 week in thissource must show non-equilibrium effects in the ionized absorber.Comment: 10 pages, 3 figures, accepted by Ap