  We present results from a 112 ks long look by XMM-Newton at the ultraluminous
X-ray source (ULX) Holmberg II X-1, long thought to be the one of best
candidates for the missing class of intermediate mass black holes (IMBHs). Our
data comprises the first high quality XMM-Newton/RGS spectrum of an ULX, and an
XMM-Newton/EPIC spectrum with unprecedented signal-to-noise. A detailed timing
analysis shows that any variability on time-scales of minutes to hours is very
weak (< few per cent fractional rms), though larger amplitude variations on
much shorter time-scales could be hidden by photon counting statistics. This
result suggests that if Ho II X-1 harbours an IMBH, then we are observing this
source in a highly unusual and atypical state when compared with the known
variability behaviour of other accreting systems of large mass. Moreover unlike
Galactic X-ray binaries, our spectral analysis indicates the possible presence
of an optically-thick low temperature corona. Taken together our timing and
spectral analysis suggests that the compact companion is most-likely a high
luminosity analogue of black hole binary systems similar to GRS 1915+105, the
Galactic microquasar, harbouring a compact object of mass no greater than 100
solar masses.
