  A method is described for the detection and estimation of transient chirp
signals that are characterized by smoothly evolving, but otherwise unmodeled,
amplitude envelopes and instantaneous frequencies. Such signals are
particularly relevant for gravitational wave searches, where they may arise in
a wide range of astrophysical scenarios. The method uses splines with
continuously adjustable breakpoints to represent the amplitude envelope and
instantaneous frequency of a signal, and estimates them from noisy data using
penalized least squares and model selection. Simulations based on waveforms
spanning a wide morphological range show that the method performs well in a
signal-to-noise ratio regime where the time-frequency signature of a signal is
highly degraded, thereby extending the coverage of current unmodeled
gravitational wave searches to a wider class of signals.
