Broadband extended emission in gravitational waves from core-collapse supernovae

Immediately following their formation, black holes in the core-collapse stage of massive stars are expected to surge in mass and angular momentum by hyper-accretion. Here we describe a general framework of extended emission in gravitational waves from non-axisymmetric accretion flows from the fallback matter of the progenitor envelope. This framework shows (a) a maximum efficiency in the conversion of accretion energy into gravitational waves at hyper-accretion rates exceeding a critical value set by the ratio of the quadrupole mass inhomogeneity and viscosity, with (b) a peak characteristic strain amplitude at the frequency f_b = Ω_b/π, where Ω_b is the Keplerian angular velocity at which viscous torques equal angular momentum loss in gravitational radiation, with h_char ∝ f^1/6 at f < f_b and h_char ∝ f^-1/6 at f > f_b. Upcoming gravitational wave observations may probe this scaling by extracting broadband spectra using time-sliced matched filtering with chirp templates, which were recently developed for identifying turbulence in noisy time series.