It is amusing that the archived ME data yield a different answer than the published result. Presumably the data and analysis software were the same. Here are large diffuse sources which, if there are no AGN within the cluster, will not vary with time. Although the emission is thermal, some clusters have high enough temperature so much of the emission is in the continuum. Figure 3 compares data for the clusters, A1796 and A401, which have been observed by many spacecraft.
Crosses show results from the literature.
These are placed at the 21cm measured value since these results do not claim to measure column density. The MPC did not measure well, and the IPC did not measure the spectral index well. Einstein data is shown for A401 to illustrate the intersection of these two results. A number of white dwarf stars emitting soft X-rays are nearby. Since these are thought not to vary, they might be good for low-energy calibrations.