A characterization of the NGC 4051 soft X-ray spectrum as observed byXMM-Newton

Abstract

<i>Context. <i/>Soft X-ray high resolution spectroscopy of obscured AGNs shows a complex soft X-ray spectrum dominated by emission lines of He and H-like transitions of elements from carbon to neon, as well as L-shell transitions due to iron ions.<i>Aims. <i/>In this paper we characterize the <i>XMM<i/>-Newton RGS spectrum of the Seyfert 1 galaxy NGC 4051 observed during a low flux state and infer the physical properties of the emitting and absorbing gas in the soft X-ray regime.<i>Methods. <i/>X-ray high-resolution spectroscopy offers a powerful diagnostic tool because the observed spectral features strongly depend on the physical properties of matter (ionization parameter <i>U<i/>, electron density <i>n<i/><sub>e<sub/>, hydrogen column density <i>N<i/><sub>H<sub/>), which in turn are tightly related to the location and size of the X-ray emitting clouds. We carried out a phenomenological study to identify the atomic transitions detected in the spectra. This study suggests that the spectrum is dominated by emission from a photoionized plasma. Then we used the photoionization code Cloudy to produce synthetic models for the emission line component and the warm absorber observed during phases of high intrinsic luminosity.<i>Results. <i/>The low state spectrum cannot be described by a single photoionization component. A multi-ionization phase gas with an ionization parameter in the range of log <i>U<i/> ~ 0.63–1.90 and a column density log <i>N<i/><sub>H<sub/> = 22.10–22.72 cm<sup>-2<sup/> is required, while the electron density <i>n<i/><sub>e<sub/> remains unconstrained. A warm absorber medium is required by the fit with the parameters log <i>U<i/> ~ 0.85, log <i>N<i/><sub>H<sub/> = 23.40 and log <i>n<i/><sub>e<sub/> 5. The model is consistent with an X-ray emitting region at a distance 5 × 10<sup>-2<sup/> pc from the central engine.