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Recent modeling of the warm absorber in active galactic nuclei has proved theusefulness of constant total (gas plus radiation) pressure models, which arehighly stratified in temperature and density. We explore the consistency ofthose models when the typical variation of the flux from the central source istaken into account. We perform a variability study of the warm absorberresponse, based on timescales and our photoionization code TITAN. We show thatthe ionization and recombination timescales are much shorter than the dynamicaltimescale. Clouds very close to the central black hole will maintain theirequilibrium since the characteristic variability timescales of the nuclearsource are longer than cloud timescales. For more distant clouds, the densitystructure has no time to vary, in response to the variations of the temperatureor ionization structure, and such clouds will show the departure from theconstant pressure equilibrium. We explore the impact of this departure on theobserved properties of the transmitted spectrum and soft X-ray variability: (i)non uniform velocities, of the order of sound speed, appear due to pressuregradients, up to typical values of 100 km/s. These velocities lead to thebroadening of lines. This broadening is usually observed and very difficult toexplain otherwise. (ii) Energy-dependent fractional variability amplitude insoft X-ray range has a broader hump around ~ 1-2 keV, and (iv) the plot of theequivalent hydrogen column density vs. ionization parameter is steeper than forequilibrium clouds. The results have the character of a preliminary study andshould be supplemented in the future with full time-dependent radiationtransfer and dynamical computations.Comment: 9 pages, 7 figures, accepted for publication by Astronomy &  Astrophysic

Response of the warm absorber cloud to a variable nuclear flux in active galactic nuclei