Systematic comparison of photoionised plasma codes with application to spectroscopic studies of AGN in X-rays

Atomic data and plasma models play a crucial role in the diagnosis and interpretation of astrophysical spectra, thus influencing our understanding of the universe. In this investigation we present a systematic comparison of the leading photoionisation codes to determine how much their intrinsic differences impact X-ray spectroscopic studies of hot plasmas in photoionisation equilibrium. We carry out our computations using the Cloudy, SPEX, and XSTAR photoionisation codes, and compare their derived thermal and ionisation states for various ionising spectral energy distributions. We examine the resulting absorption-line spectra from these codes for the case of ionised outflows in active galactic nuclei. By comparing the ionic abundances as a function of ionisation parameter $\xi$, we find that on average there is about 30% deviation between the codes in $\xi$ where ionic abundances peak. For H-like to B-like sequence ions alone, this deviation in $\xi$ is smaller at about 10% on average. The comparison of the absorption-line spectra in the X-ray band shows that there is on average about 30% deviation between the codes in the optical depth of the lines produced at log $\xi$ ~ 1 to 2, reducing to about 20% deviation at log $\xi$ ~ 3. We also simulate spectra of the ionised outflows with the current and upcoming high-resolution X-ray spectrometers, on board XMM-Newton, Chandra, Hitomi, and Athena. From these simulations we obtain the deviation on the best-fit model parameters, arising from the use of different photoionisation codes, which is about 10 to 40%. We compare the modelling uncertainties with the observational uncertainties from the simulations. The results highlight the importance of continuous development and enhancement of photoionisation codes for the upcoming era of X-ray astronomy with Athena.