The brightest ultraluminous X-ray sources across the sky

Ultraluminous X-ray sources (ULXs) can be the long-sought intermediate mass black holes (IMBHs) or stellar mass black holes. The studies of the brightest ULXs above 1e40 erg/s are most illuminating on their nature, as demonstrated by the recent discovery of a Hyperluminous X-ray source (HLXs) above 1e42 erg/s. We have carried out a search of the brightest ULXs across the sky with the ROSAT all-sky survey, which leads to a sample of 36 brightest ULXs. Six ULXs in this sample were already observed with Chandra/ACIS and all turned out to be truly bright. Here we propose to observe with Chandra/ACIS the 30 ULXs in our sample not observed before. The new observations will increase the number of HLXs above 1e42 erg/s to about ten, and triple the number of known ULXs above 1e40 erg/s.