  The disk instability picture gives a plausible explanation for the behavior
of soft X-ray transient systems if self-irradiation of the disk is included. We
show that there is a simple relation between the peak luminosity (at the start
of an outburst) and the decay timescale. We use this relation to place
constraints on systems assumed to undergo disk instabilities. The observable
X-ray populations of elliptical galaxies must largely consist of long-lived
transients, as deduced on different grounds by Piro and Bildsten (2002). The
strongly-varying X-ray source HLX-1 in the galaxy ESO 243-49 can be modeled as
a disk instability of a highly super-Eddington stellar-mass binary similar to
SS433. A fit to the disk instability picture is not possible for an
intermediate-mass black hole model for HLX-1. Other, recently identified,
super-Eddington ULXs might be subject to disk instability.
