Search for ultraluminous X-ray sources in nearby galaxies
  Little is presently known about the nature of ultraluminous X-ray sources (ULX). Different hypotheses have been proposed to explain their properties: intermediate-mass BHs, Kerr BHs, young SNR, or background AGN. Some of the current problems and open questions in this research field are here reviewed. 
L. Foschini, G. Di Cocco, L. C. Ho, L. Bassani, M. Cappi, M. Dadina,   F. Gianotti, G. Malaguti, F. Panessa, E. Piconcelli, ...