0720010

The nature of ultraluminous X-ray sources (ULXs) remains controversial. We do not know whether they host stellar- or intermediate-mass black holes nor do we understand how the accretion onto the compact object proceeds. To help answer these questions sensitive observations at energies above 10 keV are needed. We propose to observe with INTEGRAL for a total exposure of 4 Msec (2 Msec in AO7 plus another 2 Msec in the next AO) a unique region of the sky where two of the brightest known ULXs are located: M82 X-1 and Holmberg IX X-1. We will measure the spectra of these sources from 3 keV up to 50-100 keV and study their spectral variability. We will also analyze the hard X-ray spectrum of the low-luminosity AGN M81 located in the same field. [truncated! Please see actual data for full text]