X-ray Study of Ultraluminous X-ray Sources with XMM-Newton and NuSTAR

We propose to continue the simultaneous XMM+NuSTAR observing program forultraluminous Xray sources (ULXs) initiated in the previous observing cycle,with observations of two further sources of interest, Holmberg II X1 andNGC5907 ULX. The bright Xray emission from ULX binaries greatly exceeds theEddington luminosity for a spherically accreting stellarmass compact object,and understanding why this is so is one of the biggest mysteries in high energyastrophysics. Combined, XMMNewton and NuSTAR will obtain highquality spectraand timing measurements from 0.3 30 keV, providing powerful probes forunderstanding the accretion modes and nature of the central BHs. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]