Until very recently, the Galactic neutral hydrogen absorption taken into account when fitting X-ray spectra was extracted from the Dickey & Lockman (1990, hereafter DL90) survey. A better all-sky survey, the Leiden / Argentine / Bonn (LAB) Galactic H I Survey, has now become available (Kalberla et al. 2005). Though its use is not yet worldwide, it has been pointed out that, at least in some cases, the LAB survey gives notably smaller values than DL90 for the H I absorption. For example, in the case of Abell 3112, the LAB value is 1.3 × 1020 cm-2 while the DL90 value is 2.6 × 1020 cm-2, and for Sérsic 159-03 (= AS 1101), the LAB value is 1.14 × 1020 cm-2, while the DL90 value is 1.79 × 1020 cm-2. Such changes are expected to modify the soft excess emission derived from spectral fits. Fortunately, for many other clusters the differences between the H I absorption measured by LAB and by DL90 are negligible.
For Abell 3112, a fit of the XMM-Newton MOS data in a 1.5-2.9 arcmin annulus made by J. Nevalainen with a single temperature MEKAL model in the 2-7 keV band and extrapolated down to 0.3 keV using the LAB H I absorption value shows that a 20% soft excess remains (see Fig. 5). In order to eliminate the soft excess, the H I absorption should take the unrealistically low value of 1.5 1019 cm-2.
Figure 5. XMM-Newton MOS data of Abell 3112 in a 1.5-2.9 arcmin annulus and fit with a single temperature MEKAL model in the 2-7 keV band with the LAB H I absorption column, showing a 20% soft excess.
For Sérsic 159-03, new fits of the joint Suzaku and XMM-Newton data in a 3-8 arcmin annulus were made by N. Werner using the new H I LAB absorption value. If the absorption is a free parameter in the fit, NH goes to zero, and no good fit can be obtained with NH = 1.14 × 1020 cm-2. Adding a thermal component with a temperature of 0.2 keV improves the fit significantly (from a reduced χ 2 of 2.8 to 1.5), and the soft component is found to be present at a significance level of 13.5. If the fit is made with a multi-temperature model that accounts for the cluster emission with the absorption left as a free parameter, a 2 upper limit for NH of 1.4 × 1019 cm-2 is obtained; this value is inconsistent with the new LAB survey and clearly unphysically low. In the same way, if a power law is fit to the data, even if its total flux is reduced by about 20% it remains significant at a ~ 10 level. A soft component is therefore required even for zero absorption, implying that the soft excess is really strong in this cluster.
Therefore, even if they are somewhat reduced, the soft excesses observed in Abell 3112 and Sérsic 159-03 remain when taking into account the new values of the H I absorption.