Time Dependent Photoionization Modeling of Active Galactic Neclei Warm Absorbers

Warm Absorber absorption lines and edges in UV and X-ray have been extensively studied over the last few decades. However, fundamental properties like of warm absorbers: What is the mechanism which drives the outflow? What is the gas density in the flow and the geometrical distribution of the outflow? Where does the outflow originate and what is its fate? are not determined precisely and are of great interest. Progress towards understanding warm absorbers is limited by the assumption used so far in photoionization modeling: that the gas is in ionization and excitation equilibrium. This is almost certainly not correct in detail; the dynamical timescales in the flow are comparable to or less than the timescales characterizing the ionization and excitation, and the inverse processes in the gas responsible for the lines we observe. Which means the gas departs from equilibrium and hence time dependent ionization must be accounted for in the modeling of the absorber.

We have upgraded the already existing standard photoionization code XSTAR to treat non-steady state conditions. Here, we solve the coupled system of ionization balance, heating and cooling and time dependent radiative transfer equations simultaneously and self consistently. This time dependent code can model the gas in various situations, like in case of sudden changes in the luminosity of ionizing source or when the gas is receding or approaching with high-speed relative to the ionizing source.

We find that the instantaneous states of the gas do not match with any one of two steady state properties. Our work indicates that if time dependent effects are accounted for, the warm absorbers spectrum can be explained with only one component, in contrast to the two steady-state components usually found. To understand the variability of the ionizing source and the warm absorber itself, we have performed the Fourier analysis of the flux vs. time curve at different energy values and find that different ions have very different response time to the change in the flux. Detailed analysis of available X-ray spectra of warm absorbers is currently underway.