The collapse of the progenitors of core-collapse supernovae has been investigated as a source of gravitational radiation for more than three decades. In an early study published in 1971, Ruffini and Wheeler  identified mechanisms related to core collapse that could produce GWs and provided order-of-magnitude estimates of the characteristics of such emission.
Quantitative computations of GW emission during the infall phase of collapse were performed by Thuan and Ostriker  and Epstein and Wagoner [78, 77], who simulated the collapse of oblate dust spheroids. Thuan and Ostriker used Newtonian gravity and computed the emitted radiation in the quadrupole approximation. Epstein and Wagoner discovered that post-Newtonian effects prolonged the collapse and thus lowered the GW luminosity. Subsequently, Novikov  and Shapiro and Saenz [273, 253] included internal pressure in their collapse simulations and were thus able to examine the GWs emitted as collapsing cores bounced at nuclear densities. The quadrupole GWs from the ringdown of the collapse remnant were initially investigated by the perturbation study of Turner and Wagoner  and later by Saenz and Shapiro [254, 255].
Müller  calculated the quadrupole GW emission from 2D axisymmetric collapse based on the Newtonian simulations of Müller and Hillebrandt  (these simulations used a realistic equation of state and included differential rotation). He found that differential rotation enhanced the efficiency of the GW emission.
Stark and Piran [291, 238] were the first to compute the GW emission from fully-relativistic collapse simulations, using the ground-breaking formalism of Bardeen and Piran . They followed the (pressure-cut induced) collapse of rotating polytropes in 2D. Their work focused in part on the conditions for black-hole formation and the nature of the resulting ringdown waveform, which they found could be described by the quasi-normal modes of a rotating black hole. In each of their simulations, less than 1% of the gravitational mass was converted to GW energy.
Seidel and collaborators also studied the effects of general relativity on GW emission during collapse and bounce [271, 272]. They employed a perturbative approach, valid only in the slowly-rotating regime.
The gravitational radiation from non-axisymmetric collapse was investigated by Detweiler and Lindblom, who used a sequence of non-axisymmetric ellipsoids to represent the collapse evolution . They found that the radiation from their analysis of non-axisymmetric collapse was emitted over a more narrow range of frequency than in previous studies of axisymmetric collapse.
For further discussion of the first two decades of study of GW emission from stellar collapse see [87, 167, 228]. In the remainder of this section we will discuss more recent investigations.
Living Rev. Relativity 14, (2011), 1
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