  Various arguments strongly suggest that the population of ultraluminous X-ray
sources (ULXs: apparent X-ray luminosity > Eddington limit for 10 Msun ~10^39
erg/s) 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 >~10^41 erg/s). 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 AGN and tidal interactions implies that HLX activity would
switch on during passage through the host galaxy, close to pericentre. This
suggests that HLXs should appear near the host galaxy, be associated with star
formation, and thus possibly with ULXs.
