Unit 1

The first unit is the most general, in which the emission associated to the X-ray emitting component (e.g. the ICM gas) in the hydrodynamical simulation is calculated, based on a theoretical emission model. To each emitting element it is then associated a package of ideal photons, generated via a Monte-Carlo sampling of the spectral distribution.
This first unit has to be executed independently from the others, just once per simulation output.

Unit 2

The second unit of the code takes into account the geometry of the forseen observation, the observing time and the total ideal collecting area of the instrument to be used. The projection along a given line of sight is applied and photon energies are corrected for the Doppler Shift due to the l.o.s. velocity component of the parent emitting element.

Unit 3

In the third unit, the detailed instrument properties are taken into account. Here the convolution with the instrument response matrices is done.
This unit can be conveniently replaced by any desired X-ray instrument simulator, like XISSIM for the Suzaku X-ray Imaging Spectrometer, MARX for Chandra, or SIXTE, designed to obtain mock observations for eROSITA, ATHENA or XMM-Newton.

ICM X-ray emission. The spectral model assumed can be very complicated and account for various thermal and chemical properties of the emitting gas, which are computed in state-of-the-art hydro-simulations. These include not only temperature and density, but also total metallicity or detailed chemical composition and sub-grid turbulent velocity which causes the non-thermal broadening of X-ray emission lines.

X-ray emission from AGNs. Unit 1 has also been modified to account for X-ray emission from AGN sources in addition to the ICM, for simulations where the evolution of BH elements is treated.
Biffi et al. 2018 (MNRAS 481, 2213)