  Signatures of warm absorbers are seen in soft X-ray spectra of about half of
all Seyfert1 galaxies observed and in some quasars and blazars. We use the
thermal equilibrium curve to study the influence of the shape of the ionizing
continuum, density and the chemical composition of the absorbing gas on the
existence and nature of the warm absorbers. We describe circumstances in which
a stable warm absorber can exist as a multiphase medium or one with continuous
variation in pressure. In particular we find the following results: i) the warm
absorber exists only if the spectral index of the X-ray power-law ionizing
continuum $\alpha > 0.2$ and has a multiphase nature if $\alpha \sim 0.8$,
which interestingly is the spectral index for most of the observed Seyfert 1
galaxies; ii) thermal and ionization states of highly dense warm absorbers are
sensitive to their density if the ionizing continuum is sufficiently soft, i.e.
dominated by the ultraviolet iii) absorbing gas with super-Solar metallicity is
more likely to have a multiphase nature; iv) the nature of the warm absorber is
significantly influenced by the absence of iron and associated elements which
are produced in the later stages of star formation history in supernovae of
type Ia.
