Detection of collapse GW signals

5.1 Detection of collapse GW signals

Let’s review some of the GW sources and mechanisms discussed in this paper. Figure 32

shows the GW signal from matter motions from bounce and convection in normal stellar collapse assuming a source at 10 kpc (within the Milky Way). The strong signals presented here are upper limits, assuming asymmetries and stellar spin rates that are greater than the values that are produced in the current best-estimates from stellar models (current models are better at placing upper limits than quantitative results). The corresponding signal from neutrinos is shown in Figure 33

. For asymmetric collapse, the neutrino-induced GW signal may dominate the signal detected by LIGO. But, in general, it is likely that the matter-induced signal will dominate the signal observed by LIGO.

Figure 32: GW signals from matter motions in the bounce and convection phases of stellar collapse. These limits and estimates have not changed from the previous publication of this Living Reviews article [111

]. Uncertainties in the estimates still allow variations within an order of magnitude of these results. The LIGO and advanced LIGO sensitivities are included for reference. The limits are not upper limits of a single signal, but upper limits of all possible signals (varying the conditions of the collapse). This figure does not represent a single signal.

Figure 33: GW signals from asymmetric neutrino emission in the bounce and convection phases of stellar collapse. These limits and estimates have not changed from the previous publication of this Living Reviews article [111 ]. Uncertainties in the estimates still allow variations within an order of magnitude of these results. The LIGO and advanced LIGO sensitivities are included for reference. The limits are not upper limits of a single signal, but upper limits of all possible signals (varying the conditions of the collapse). This figure does not represent a single signal.

In general, the signal at bounce is strongest for the rotating (and most asymmetric) explosions. The strong bounce signal is possible in AICs, low-mass collapse and normal core-collapse supernovae. The rest of the signals in Figures 32 and 33 are only appropriate for normal core-collapse supernovae. Except in the fastest-rotating cases, the signals from AICs, low-mass collapse and normal supernovae are only observable if they occur within local group galaxies. We are unable to observe stellar collapse in the nearby Virgo cluster. A detection of a stellar collapse in this cluster would argue that either extreme rotation does occur in stars or that bar modes exist in these cores.

If bar modes do develop in the proto neutron star, the GW signal may be orders of magnitude higher amplitude than that produced in the bounce or convective phase. Figure 23 shows the signal for bar-mode sources assuming the source is at 10 Mpc instead of the 10 kpc. Dynamical bar modes could be easily detected out to the Virgo cluster. Even secular bar modes should be detected out to Virgo. If such modes are produced in even 1/10th of all stellar collapses, advanced LIGO should detect multiple GW outbursts per year as soon as it reaches its design specifications. Non-detections place limits on the spin rate of stars.

Black hole forming systems have the potential to form stronger GWs. But the fragmentation claimed in many results did not occur in the 3-dimensional models by Rockefeller et al. [250 ]. Their predicted GW signal was on par with the upper limits to rotational collapse shown in Figure 32 . Like normal stellar collapse, black-hole–forming objects are unlikely to be observed as far out as the Virgo cluster.

Although the signal for very massive stars ( $∼ 300M ⊙$ ) is expected to be much larger than other black-hole–forming systems, these objects are believed to only form in the early universe (modest metallicities cause these stars to lose most of their mass in winds, in case they behave more like lower-mass black-hole–forming systems). At such distances, these collapses will be difficult to observe. However, if the rate of these objects is high, it may be that future detectors such as the DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) and the Big Bang Observer (BBO) may be able to detect GW emission from these objects [298 ].

The possible exception for detectable GW emission from black-hole formation is the formation of SMBHs. SMBHs exist. If they are formed in the collapse of an SMS, a number of observational sources should allow us to observe these objects with LISA (Figure 30 ).