Searching for IMBH candidates among Ultraluminous X-ray Sources

A subsample of bright Ultraluminous Xray Sources (L_Xvirgul(15)x10^40 erg/s) showspeculiar powerlaw like spectra, with no high energy turnoff and shorttermvariability at the level of virgul10%. These sources might host Intermediate MassBlack Holes (IMBHs) in a low/hard state, although their apparently flat spectralshape may be the effect of poor counting statistics. Here we proposesimultaneous, deep XMM Newton and NuSTAR observations of four such objects tofind evidence either of the Xray properties (powerlaw spectra and short termvariability) that should characterize subEddington accretion onto an IMBH or ofthe characteristic spectral curvature of the Ultraluminous state.. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]