SIMBAD references

In about half of the Seyfert galaxies, the X-ray emission is absorbed by an optically thin, ionized medium, the so-called ``warm absorber'', whose origin and location is still a matter of debate. The aim of this paper is to constrain the warm absorber further by studying its variability. We analyzed the X-ray spectra of a Seyfert 1 galaxy, Mrk 704, which was observed twice, three years apart, by XMM-Newton. The spectra were well fitted with a two-zones absorber, which possibly covers the source only partially. The parameters of the absorbing matter - column density, ionization state, covering factor - changed significantly between the two observations. Possible explanations for the more ionized absorber are a torus wind (the source is a polar scattering one) or, in the partial covering scenario, an accretion disk wind. The less ionized absorber is possibly composed of orbiting clouds in the surroundings of the nucleus, similarly to what was already found in other sources, most notably NGC 1365.