Constraining the Cosmic Background

The RASS spectrum constraint can significantly improve the reliability of the fits. The middle panel of Figure 20 shows how the initial fit (upper panel) with only the XMM-Newton data is a poor representation as it significantly under predicts the cosmic background at lower energies for the RASS data. To do so, and to produce the apparently good fit of the upper panel, power has been shuffled between model components. Using the RASS data to constrain the fit forces the power to be shifted to (hopefully) the proper components, as is shown in the lower panel of Figure 20.

Figure 21: Radial profiles of the fitted temperatures for the annuli of the Abell 1795 cluster. Green points are from the fits to the data without adding the RASS data as a constraint. Red points show the fitted values when the RASS data are simultaneously fit. The annuli used are the same but were given shifted radii for clarity. The blue points are from refitting the data applying the crosstalk ARFs to account for flux from one region of the sky which due to the finite PSF appear in a different region of the detector.

Figure 21 shows how the application of the RASS constraint can affect the fitted parameters. It shows the fitted values for the temperature radial profile of Abell 1795. As expected near the center there is very little difference since the cluster emission so dominates the background. However, at large radii where the cluster emission isn't so bright, the fitted values for the temperature start to vary significantly.

The final background component that can play a significant role in spectral analysis is the solar wind charge exchange flux. As noted above, this is particularly problematic as in the XMM-Newton energy range it is primarily comprised of emission lines that are typically of astrophysical interest, e.g., O VII, O VIII, Ne IX, and Mg XI. Also as noted above, occasionally the presence of SWCX emission can be seen by comparing the $0.5-0.7$ keV and $2.0-8.0$ keV light curves and searching for variations in the former that are not seen in the latter. However, intervals as long as 30 ks have been observed where the SWCX contamination is significant and yet the light curve has remained relatively constant. Another symptom of the contamination is the detection of very strong O VIII and Mg XI lines. However, as there are astrophysical objects that also show enhanced emission in these lines some care must be taken with the interpretation of the results. In some cases it may be advantageous to include additional Gaussian lines for the O VII, O VIII, and perhaps other lines.