Where are they?
Found in 20% of spiral galaxies
40% of ellipticals (7
in Fornax A, 6 in NGC 1553)
most tidally-disrupted starburst (10 in Antennae)
Very old populations
Very young populations
Variability?
All are persistent
Most variable by a factor of a few (over
hours/yrs)
Long duty cycle? If so, how
many quiescent sources?
State transitions?
Some
similarities with Galactic BH (Roberts et al 2000)