5 X-ray colour-colour diagrams

The unprecedented combination of high throughput and wide energy band makes XMM uniquely suited to classify sources based on their X-ray spectra alone. The large number of X-ray sources in the Lockman Hole which already have spectroscopic identifications and redshifts based on the ROSAT surveys can be used as a training set for the classification of the still unidentified XMM sources in the Lockman Hole, but also in other XMM fields.

From the count rates in the four independent energy bands used for source detection (see Sect. 4) we have calculated three statistically independent hardness ratios, HR1, HR2 and HR3, according to the formula HR = (H-S)/(H+S), where H and S correspond to the counts in the harder and softer energy bands, respectively (see Table 2). Figure 4 shows two X-ray colour-colour diagrams with different symbols for various source classes superposed on a model grid for absorbed power law spectra. The type-1 AGN known from ROSAT populate a narrow, relatively soft range in both diagrams, consistent with a photon index of $\Gamma \approx 2$ and typically low apparent absorption column densities ( $\log(N_{\rm H})<21.5~{\rm cm}^{-2}$). The new XMM sources, together with a number of previously known type-2 ROSAT AGN scatter over a much wider area. Although the new XMM sources are typically fainter than the ROSAT sources and therefore have larger hardness ratio errors, the new population is considerably harder in both diagrams. A comparison of the source colours with the underlying model grids shows, that the hardening is mainly due to apparent absorption column densities of $\log(N_{\rm H})>21.5~{\rm cm}^{-2}$ on top of relatively soft spectra and not due to intrinsically hard power law indices. Some sources fall outside the model grid. In one case, a ROSAT Seyfert-2 galaxy which shows the largest HR3, this is probably due to a very soft component on top of a heavily absorbed power law. In some cases the hardness ratios might be affected by source confusion. A similar, but more extended analysis has been applied to the broad-band spectral properties of the ASCA sources by Della Ceca et al. (1999).