Broadband extended emission in gravitational waves from core-collapse supernovae

Black holes in core-collapse of massive stars are expected to surge in mass and angular momentum by hyper-accretion immediately following their formation. We here describe a general framework of extended emission in gravitational waves from non-axisymmetric accretion flows from fallback matter of the progenitor envelope. It shows (a) a maximum efficiency in 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 $f_b=\Omega_b/\pi$, where $\Omega_b$ is the Keplerian angular velocity at which viscous torques equal angular momentum loss in gravitational radiation, with $h_{char}\propto f^{1/6}$ at $f<f_b$ and $h_{char}\propto f^{-1/6}$ at $f>f_b$. Upcoming gravitational wave observations may probe this scaling by extracting broadband spectra using time-sliced matched filtering with chirp templates, recently developed for identifying turbulence in noisy time series.