Not yet reviewed by Pith; the record is open.

Photoionisation is one of the main mechanisms at work in the gaseous environment of bright astrophysical sources. Many information on the gas physics, chemistry and kinematics, as well as on the ionising source itself, can be gathered through optical to X-ray spectroscopy. While several public time equilibrium photoionisation codes are readily available and can be used to infer average gas properties at equilibrium, time-evolving photoionisation models have only very recently started to become available. They are needed when the ionising source varies faster than the typical gas equilibration timescale. Indeed, using equilibrium models to analyse spectra of non-equilibrium gas may lead to inaccurate results and prevents a solid assessment of the gas density, physics and geometry. We present our novel Time-Evolving PhotoIonisation Device (TEPID), which self-consistently solves time evolving photoionisation equations (thermal and ionisation balance) and follows the response of the gas to changes of the ionising source. The code can be applied to a variety of astrophysical scenarios and produces time-resolved gas absorption spectra to fit the data. To describe the main features of TEPID, we apply it to two dramatically different astrophysical scenarios: a typical ionised absorber observed in the X-ray spectra of Active Galactic Nuclei (e.g. Warm Absorbers and UFOs) and the circumburst environment of a Gamma-Ray Burst. In both cases, the gas energy and ionisation balances vary as a function of time, gas density and distance from the ionising source. Time evolving ionisation leads to unique ionisation patterns which cannot be reproduced by stationary codes when the gas is out of equilibrium. This demonstrates the need for codes such as TEPID in view of the up-coming high-resolution X-ray spectrometers onboard missions like XRISM or Athena.