  The brightest events in a time series of cosmological transients obey an
observation time dependence which is often overlooked. This dependence can be
exploited to probe the global properties of electromagnetic and gravitational
wave transients (Howell et al. 2007a, Coward & Burman 2005). We describe a new
relation based on a peak flux--observation time distribution and show that it
is invariant to the luminosity distribution of the sources (Howell et al.
2007b). Applying this relation, in combination with a new data analysis filter,
to \emph{Swift} gamma-ray burst data, we demonstrate that it can constrain
their rate density.
