Context : Many active galactic nuclei exhibit X-ray features typical of the highly ionized gas called “ Warm Absorber ” ( WA ) .
Such a material appears to be stratified , displaying zones of different density , temperature , and ionization .
In this paper , we investigate the possibility of modelling the WA gas in NGC 3783 as a single medium in total pressure equilibrium .

Aims : Our goal is to demonstrate that the WA can be well modelled assuming constant total pressure , in contrast to the current descriptions that are based on the presence of multiple regions , each in constant density .
The assumption of total pressure equilibrium yields a more physical description of the WA , resulting in the natural stratification of the ionized gas , and providing an explanation for the presence of lines from different ionization states , as observed in WA spectra .

Methods : We have used the photoionization code TITAN , developed by our team , to compute a grid of constant total pressure models with the purpose of fitting the WA in NGC 3783 .
We have compared our models to the 900 ks Chandra spectrum of NGC 3783 and to previous studies where the WA was described by multiple zones of constant density .

Results : In the case of NGC 3783 , the WA features can be well reproduced by a clumpy , ionized gas with cosmic abondances , ionization parameter \xiup = 2500 ~ { } \mathrm { erg cm s ^ { -1 } } , column density { N _ { H } } = 4 10 ^ { 22 } ~ { } \mathrm { cm ^ { -2 } } , and constant total pressure .

Conclusions : We have shown that the WA in NGC 3783 can be modelled by a single medium in total pressure equilibrium ; this is probably the case for other WAs currently described by multi-zone , constant density models .
In addition , our work demonstrates that the TITAN code is well adapted to the study of the WA in active galactic nuclei , opening new prospects for the use of TITAN by a larger community .