tokens: the MOS 1, MOS 2, and pn data with the xspec model const*phabs ( cemekl ) (see text for details). 2.4 RGS data Note that the same multitemperature model used for the EPIC spectral fit provides an adequate description of the RGS spectra when fixing all the parameters of the model to those derived above but adjusting consistently the model normalizations. In particular, inspecting Fig. 5 (where the best fit is superimposed to the RGS1 and RGS2 data re-binned in order to have at least a significance of 10σ per bin), one can note the existence of the O VIII Ly α emission line at ≃ 0.65 keV, and emission lines in the energy range 0.56–0.57 keV (21.6–22.1 ) possibly associated with the He-like transitions of O VII , i.e. the resonance line r (corresponding to the transitions between the n = 2 shell and the n = 1 ground state), and the intercombination ( i ) and the forbidden ( f ) lines. In this respect, as shown by Porquet Dubau ( 2000 ) the relative emission strength of the r, i , and f lines is a good indicator of the physical conditions of density and temperature of the gas. Figure 5. The best-fitting model superimposed on the RGS 1 (red) and RGS 2 (black) data. Note the presence a strong O VIII Ly α line at 18.96 and the O VII He-like triplet. In order to have a measure of the line fluxes, we followed the phenomenological spectral analysis method described in Nucita et al. ( 2010 ) and references therein. In particular, the unbinned RGS spectra are divided in intervals of 100 channels wide and Gaussians are fitted to all the identified emission lines. For each (single) emission line the centroid energy is free to vary and, when triplets are identified, the relative distance between the central energies was frozen to the value predicted by atomic physics. The local continuum was always modelled as a power law with a fixed photon index Γ = 1 and normalization free to vary. Since we are dealing with the unbinned spectra, we estimated the goodness of the fit by using the C-statistic (see Cash 1979 ). We further consider the line (or triplet) detected at confidence level when, repeating the fit with the continuum only, we obtained a change in the C-statistic value (Δ C ) by at least 2.3. The result of this analysis (see also Table 2 ) showed that the O VII resonance line is much more weaker than the intercombination line so that ionization processes may occur in the medium (Porquet Dubau 2000 ). This is also clear when using the standard line ratios R = f / i, L = r / i , and G = ( f + i )/ r as standard diagnostic. In particular, as clear from table 2, the large (small) values of the G ( R ) ratio imply photoionization and, since R decreases with increasing values of the electron density n e , a lower limit on the electron density of ≃10 11 cm −3 (Porquet Dubau 2000 ). Table 1. Spectral parameters for the best-fitting model ( const * phabs *( cemekl ) in xspec , see text for details). We also give the 0.3–10 keV absorbed and unabsorbed fluxes.
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