A new gravitational-wave signature of low-T/|W| instability in rapidly rotating stellar core collapse

We present results from a full general relativistic three-dimensional hydrodynamics simulation of rapidly rotating core collapse of a 70 M_⊙ star with three-flavour spectral neutrino transport. We find a strong gravitational-wave (GW) emission that originates from the growth of the one- and two-armed spiral waves extending from the nascent proto-neutron star (PNS). The GW spectrogram shows several unique features that are produced by the non-axisymmetric instabilities. After bounce, the spectrogram first shows a transient quasi-periodic time modulation at ∼450 Hz. In the second active phase, it again shows the quasi-periodic modulation but with the peak frequency increasing with time, which continues until the final simulation time. From our detailed analysis, such features can be well explained by a combination of the so-called low-T/|W| instability and the PNS core contraction.