Variability in Ultra-luminous X-ray Sources
  Many upcoming surveys, particularly in the radio and optical domains, are
designed to probe either the temporal and/or the spatial variability of a range
of astronomical objects. In the light of these high resolution surveys, we
review the subject of ultra-luminous X-ray (ULX) sources, which are thought to
be accreting black holes for the most part. We also discuss the sub-class of
ULXs known as the hyper-luminous X-ray sources, which may be accreting
intermediate mass black holes. We focus on some of the open questions that will
be addressed with the new facilities, such as the mass of the black hole in
ULXs, their temporal variability and the nature of the state changes, their
surrounding nebulae and the nature of the region in which ULXs reside.
