One of the problems for the study of diffuse emission with XMM-Newton is that several of the important components have a significant fraction of their emission just outside the XMM-Newton band pass. The Local Hot Bubble has the bulk of its emission in the keV band, and the same is true of the softer components of the Galactic halo. Thus, XMM-Newton-only spectral fits do not constrain these components well, despite the fact that there is still a significant amount of emission from them in the XMM-Newton band pass. These considerations are not usually important for the spectral fitting needed for imaging analysis!
One way of helping to constrain the fits for these components is to use the spectral energy distribution (i.e. a very low resolution spectrum) from the ROSAT All-Sky Survey (RASS). The RASS data has an effective angular resolution of 12 but has a sufficiently low count rate that one needs about a square degree of data to get a good signal-to-noise ratio. The RASS data is in seven bands over 0.11 to 2.X keV, so it overlaps XMM-Newton band pass nicely. This overlap allows one to see whether the ROSAT data, being taken from a somewhat larger region than an XMM-Newton FOV, is consistent with the XMM-Newton data. Use of the ROSAT SED with XMM-Newton spectra is further demonstrated in §6.7.1.
The RASS SED can be obtained using the HEASARC X-ray background tool: https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/xraybg/xraybg.pl
One issue with using the RASS SED to fit the XMM-Newton spectra is that of the SWCX. The bulk of the SWCX emission in the RASS was due to magnetospheric/exospheric emission, which was manifested as the long-term enhancements (LTEs), which were removed before the release of the RASS. Witihn the RASS there remains not only residual LTE emission, but also a certain amount of heliospheric SWCX emission. The work of Liu et al (2017) shows that the heliospheric contribution is relatively small.
There are a number of reasons not to attempt to fit a SWCX component in the RASS data. First, although we know that the SWCX at the time of the RASS observations will be different from that during the XMM-Newton observations, we do not have the information required to calculate the SWCX emission during the RASS. Further, we don't yet have a robust model for the SWCX emission within the keV band. Second, the SWCX emission remaining in the RASS SED is small, and is lost in the only moderate signal-to-noise ratio of the RASS SED. However, for the most precise work, one would want to correct the RASS SED for the heliospheric emission found by Lie et al (2017).