Limited options in the interpretation of XMM cluster soft excess data

By using improved background subtraction routines and simultaneous modeling of XMM/Newton, ROSAT and EUVE data, we discriminate between thermal and power-law fits of the soft excess for all regions of several clusters. It will be shown that for the inner soft excesses only the non-thermal model involving a large pressure of cosmic rays is viable- the physical implications of such a relativistic population, especially on the cooling flow process, will be addressed. The outer soft excesses cannot be caused by cosmic rays, because of obvious confinement problems, yet the thermal interpretation depends crucially on the reality of the O VII lines because the WHIM model of Cen & Ostriker predicts a soft flux that falls short of observations by ∼ 100 times. In this presentation emphasis will be made of the fact that, as one full decade since discovery approaches, there is still no firm explanation of this new radiation component from the largest organized structures in the Universe. With the advent of XMM/Newton, however, we do know with certainty that this excess is real.