The field of gravitational-wave (GW) astronomy will soon become a reality. The first generation of ground-based interferometric detectors (LIGO , VIRGO , GEO600 , TAMA300 ) are beginning their search for GWs. Upgrades for two of these detectors (LIGO, VIRGO) have already begun and in the next few years, the sensitivity of both these detectors will be increased. In addition, these two detectors have begun working more closely together, improving their net sensitivity. After these detectors are complete, they will have sensitivities necessary to regularly detect emissions from astrophysical sources. New detectors, such as the Large-Scale Cryogenic Gravitational-wave Telescope (LCGT) as well as technology developing sites such as the Australian Interferometric Gravitational Observatory (AIGO) are also contributing to the progress in GW astronomy (see  for a review). A space-based interferometric detector, LISA , could be launched in the next decade. One important class of sources for these observatories is stellar gravitational collapse. This class covers an entire spectrum of stellar masses, from the accretion-induced collapse (AIC) of a white dwarf and the collapse of low-mass stars, including electron-capture supernovae (), through the collapse of massive stars () that produce and the even more massive stars () that produce the “collapsar” engine believed to power long-duration gamma-ray bursts , massive and very massive Population III stars (), and supermassive stars (SMSs, ). Some of these collapses result in explosions (Type II, Ib/c supernovae and hypernovae) and all leave behind neutron-star or black-hole remnants1.
Strong GWs can be emitted during a gravitational collapse/explosion and, following the collapse, by the resulting compact remnant [309, 205, 206, 88, 269, 108, 150]. GW emission during the collapse itself may result if the collapse or explosion involves aspherical bulk mass motion or convection. Rotational or fragmentation instabilities encountered by the collapsing star will also produce GWs. Pulsations and instabilities in the newly formed neutron star (a.k.a. proto neutron star) may also produce observable GWs. Asymmetric neutrino emission can also produce a strong GW signature. Neutron-star remnants of collapse may emit GWs due to the growth of rotational or r-mode instabilities. Black-hole remnants will also be sources of GWs if they experience accretion-induced ringing or if the disks around the black hole develop instabilities. All of these phenomena have the potential of being detected by GW observatories because they involve the rapid change of dense matter distributions.
Observation of gravitational collapse by GW detectors will provide unique information, complementary to that derived from electromagnetic and neutrino detectors. Gravitational radiation arises from the coherent superposition of mass motion, whereas electromagnetic emission is produced by the incoherent superposition of radiation from electrons, atoms, and molecules. Thus, GWs carry different kinds of information than other types of radiation. Furthermore, electromagnetic radiation interacts strongly with matter and thus gives a view of the collapse only from lower density regions near the surface of the star, and it is weakened by absorption as it travels to the detector. In contrast, gravitational waves can propagate from the innermost parts of the stellar core to detectors without attenuation by intervening matter. With their weak interaction cross-sections, neutrinos can probe the same region probed by GWs. But whereas neutrinos are extremely sensitive to details in the microphysics (equation of state and cross-sections), GWs are most sensitive to physics driving the mass motions (e.g., rotation). Combined, the neutrino and the GW signals can teach us much about the conditions in the collapsing core and ultimately the physics that governs stellar collapse (e.g., [7, 107, 167, 228]).
The characteristics of the GW emission from gravitational collapse have been the subject of much study. Core-collapse supernovae, in particular, have been investigated as sources of gravitational radiation for nearly four decades (see, e.g., [251, 311, 253, 64, 204, 89, 201, 350, 245, 106, 108, 67, 68, 165, 232]). However, during this time research has produced estimates of GW strength that vary over orders of magnitude. This is due to the complex nature of core collapse. Important theoretical and numerical issues include
To date, collapse simulations generally include state-of-the-art treatments of only one or two of the above physics issues (often because of numerical constraints). For example, those studies that include advanced microphysics have often been run with Newtonian gravity (and approximate evaluation of the GW emission; see, e.g., Section 4.1). Very few, if any simulations, have reached any convergence in spatial resolution. Many of the codes have not been tested to see if their algorithm implementations guard against standard numerical artifacts. For example, very few codes used have tested the effects of the non-conservation of angular momentum and the numerical transport of this angular momentum. A 3D, general relativistic collapse simulation that includes all significant physics effects is not feasible at present. However, good progress has been made on the majority of the issues listed above; the more recent work will be reviewed in some detail here.
The remainder of this article is structured as follows. We first review the basic modes of GW emission in stellar collapse presenting, where they exist, analytic formulae that have been used to estimate these GWs (Section 2). The latter half of Section 2 presents many of the numerical approximations used to calculate GW emission. Section 3 covers the various collapse scenarios and their GW sources: normal core-collapse supernovae (Section 3.1), the accretion induced collapse (AIC) of a white dwarf (Section 3.2), the collapse of low mass stars and electron capture supernovae (Section 3.3), and the collapse of massive (Section 3.4) and supermassive (Section 3.5) stars. For each, we review the current understanding of the occurrence rate, collapse evolution, and the specific causes behind GW emission. Section 4 then discusses the current state of the calculations for sources arising from: bounce (Section 4.1), convection (Section 4.2), bar modes (Section 4.3), neutrinos (Section 4.4), r-modes (Section 4.5), fragmentation (Section 4.6), and ringing (Section 4.7). We conclude by tying together all of these sources with their emission mechanisms to predict a complete GW sky.
One final word of warning. In many cases, the total GW signal from stellar collapse can be tuned by altering key initial conditions (such as the rotation rate of the collapsing star). Many of the strongest GW estimates in the literature tend to use rotation rates that are orders of magnitude higher than that predicted for most stars. These more optimistic results often predict that the current set of detectors should observe GWs from astrophysical sources. In many cases, studies of these extreme conditions provide insight into possible mechanisms for GW emission. To include these new insights, we will discuss these results in this review. The exact nature of the initial conditions may make certain GW signals undetectable. For each of the these scenarios, we strive to distinguish academic studies with more realistic estimates of the signal. Our summary is based on what we judge to be the more realistic signal predictions.
Living Rev. Relativity 14, (2011), 1
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