UNVEILING BLACK HOLES AND NEUTRON STARS IN ULXs

A subsample of bright ULXs (Lxvirgul(15)e40 erg/s) shows peculiar powerlaw likespectra in the 0.3 10 keV band, with no high energy turnoff and shorttermvariability at the level of virgul10%. These sources might host Intermediate MassBlack Holes (IMBHs) in a hard state, although their apparently flat spectra maybe the effect of low counting statistics. However, hard states with low energyspectral cutoff can indicate the presence of superEddington accreting neutronstars or stellar mass BHs. Here we propose simultaneous, deep XMMNewton andNuSTAR observations of three best candidates to find evidence either of IMBHs,accreting subEddington and with hard powerlaw spectra and high short termvariability, or of the characteristic spectral shape of the superEddington accretion. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]