A characterization of the NGC 4051 soft X-ray spectrum as observed by XMM-Newton RID B-4804-2010

Context. 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. Aims. In this paper we characterize the XMM-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. Methods. 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 U, electron density ne, hydrogen column density N(H)), 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. Results. 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 U similar to 0.63-1.90 and a column density log N(H) = 22.10-22.72 cm(-2) is required, while the electron density ne remains unconstrained. A warm absorber medium is required by the fit with the parameters log U similar to 0.85, log N(H) = 23.40 and log n(e) less than or similar to 5. The model is consistent with an X-ray emitting region at a distance greater than or similar to 5 x 10(-2) pc from the central engine.