Hierarchical merging, ultraluminous and hyperluminous X-ray sources

Various arguments strongly suggest that the population of ultraluminous X- ray sources (ULXs, apparent X- ray luminosity > Eddington limit for 10 M. similar or equal to 10(39) erg s(-1)) in nearby galaxies are mostly stellar- mass X- ray binaries in unusual evolutionary stages. However, there are indications that the very brightest systems may be difficult to explain this way. Accordingly, we consider the class of hyperluminous X- ray sources (HLXs; i. e. those with apparent bolometric luminosities greater than or similar to 10(41) erg s(-1)). Because this class is small (currently only the M82 object is a secure member) we do not need to invoke a new formation mechanism for its black holes. We explore instead the idea that HLXs may be the nuclei of satellite galaxies captured during hierarchical merging. The observed correlation between active galactic nuclei and tidal interactions implies that HLX activity would switch on during passage through the host galaxy, close to the pericentre. This suggests that HLXs should appear near the host galaxy, be associated with star formation and thus possibly with ULXs.