  The BATSE catalogue is searched for evidence of spindown of black holes or
proto-neutron stars (PNS) by extracting normalized light curves (nLC). The nLC
are obtained by matched filtering, to suppress intermediate time scales such as
due to the shock break-out of GRB jets through a remnant stellar envelope. We
find consistency within a few percent of the nLC and the model template for
spindown of an initially extremal black hole against high-density matter at the
ISCO. The large BATSE size enables a study of the nLC as a function of
durations $T_{90}$. The resulting $\chi^2_{red}$ is within a $2.35\sigma$
confidence interval for durations $T_{90}>20$ s, which compares favorably with
the alternative of spindown against matter further out and spindown of a PNS,
whose $\chi^2$ fits are, respectively, outside the 4$\sigma$ and 12$\sigma$
confidence intervals. We attribute spindown against matter at the ISCO to
cooling by gravitational-wave emission from non-axisymmetric instabilities in
the inner disk or torus as the result of a Hopf bifurcation in response to
energetic input from the central black hole. This identification gives an
attractive outlook for chirps in quasi-periodic gravitational waves lasting
tens of seconds of interest to LIGO, Virgo and the LCGT.
