5.2 Using GWs to study core collapse

Beyond the detection of GWs, GW observations of stellar collapse allow us to probe the mechanism behind core-collapse explosions. Along with neutrinos, GWs provide one of the few windows into the heart of the core-collapse supernova explosion mechanism. As we have already discussed, the detection (or lack thereof) of a GW signal can tell us something about the spin rate of the core. For instance, a strong signal from a supernova at the Virgo cluster would require bar-mode instabilities and, hence, a rotation rate in the core that is much faster than we currently believe. Likewise, a strong GRB GW signal would imply fragmentation in the disk that we, as yet, do not observe. This would lead to a major rethinking in the GRB engine.

Scientists also argue that the strength and form of the GW signal in a galactic supernova would allow us to distinguish between asymmetric and rotating collapse and determine the magnitude of rotation (or degree of asymmetry) in these collapse models (e.g., [107, 228, 297Jump To The Next Citation Point]). If we knew the theoretical signal unambiguously, scientists could determine the progenitor rate and degree of differential rotation [297]. Unfortunately, at this time, the range of results discussed in this review is too large for such a study; it points out the potential for GW to address very specific astronomy questions if collapse models reach some agreement on the exact GW signal.

But precision measurements in gravitational collapse have been hampered by the lack of true verification and validation studies of the existing codes. Although the amount of angular momentum in the core is critical to determining the exact wave signal, very few solid tests of angular momentum conservation (and artificial angular-momentum transport) have been made on most codes. This issue is even more important for three-dimensional calculations. Until this source of error can be quantified and minimized, the errors in the GW signal will remain large, stagnating this field and limiting what we can learn from an observed signal.

The detection of GWs from weak or dim supernovae will provide clues to these rarely-observed classes of supernovae. Dim supernovae can be produced both by the collapse of low-mass stars (electron-capture supernovae) or the collapse of massive stars that have considerable fallback. Models argue that these explosions are difficult to observe and such supernovae may occur nearby without detection in the optical. GWs may provide the only direct means to study these supernovae. Similarly, different GRB models predict very different GW signals and the GW detection of a GRB will place strong constraints on the current engine models.

Finally, detectors such as LIGO will build up a sample of merging systems consisting of neutron stars or black holes. From these detections, we can derive strong observational constraints on the birth mass distributions of both neutron stars and stellar-massed black holes. This set of observations will indirectly constrain the explosion mechanisms for core collapse.

Current studies have begun to probe the tip of the iceberg of what we will be able to study in core collapse with GWs and we expect the number of papers probing this topic to increase dramatically in the next few years.


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