Core-collapse astrophysics with a five-megaton neutrino detector

The legacy of solar neutrinos suggests that large neutrino detectors should be sited underground. However, to instead go underwater bypasses the need to move mountains, allowing much larger water Čerenkov detectors. We show that reaching a detector mass scale of $\sim 5$ Megatons, the size of the proposed Deep-TITAND, would permit observations of neutrino “mini-bursts” from supernovae in nearby galaxies on a roughly yearly basis, and we develop the immediate qualitative and quantitative consequences. Importantly, these mini-bursts would be detected over backgrounds without the need for optical evidence of the supernova, guaranteeing the beginning of time-domain MeV neutrino astronomy. The ability to identify, to the second, every core collapse in the local Universe would allow a continuous “death watch” of all stars within $\sim 5\mathrm{Mpc}$, making practical many previously-impossible tasks in probing rare outcomes and refining coordination of multiwavelength/multiparticle observations and analysis. These include the abilities to promptly detect otherwise-invisible prompt black hole formation, provide advance warning for supernova shock-breakout searches, define tight time windows for gravitational-wave searches, and identify “supernova impostors” by the nondetection of neutrinos. Observations of many supernovae, even with low numbers of detected neutrinos, will help answer questions about supernovae that cannot be resolved with a single high-statistics event in the Milky Way.

Figure 1

Probabilities to obtain the indicated numbers of ${\overline{\nu }}_e$ neutrino events (with $E_{e^{+}}>18\mathrm{MeV}$) in a 5 Mton detector as a function of the supernova distance. We assume a Fermi-Dirac ${\overline{\nu }}_e$ spectrum with an average energy of 15 MeV and a total energy of $5\times {10}^{52}\mathrm{erg}$. Optical supernovae observed in the 10 years from 1999–2008 are noted at their distances, with four galaxies hosting multiple supernovae (in light grey; see also Table ).

Figure 2

Estimates of the core-collapse supernova rate in the nearby universe, based on that expected from the optical luminosities of known galaxies (line) and 22 supernovae observed in 1999–2008 (bins). Note that SN 2002kg is a likely LBV outburst, while SN 2008S and the NGC 300 transient are of unusual origin. These estimates are all likely to be incomplete.

Figure 3

Probabilities for one or more supernovae in the Milky Way over time spans relevant for the lifetimes of large neutrino detectors, depending on the assumed supernova rate.

Figure 4

Frequency of neutrino mini-bursts expected with a 5 Mton detector. The bins with $N=3$ or more can be used for burst detection because the background rate is small enough. Three different estimates of the supernova rate are shown, as labeled.

Figure 5

Relative number of $N\ge 3$ neutrino mini-burst detections and summed neutrino counts as the expected neutrino event yield in Eq. (1) is varied from our fiducial case of a Fermi-Dirac ${\overline{\nu }}_e$ spectrum with an average energy of 15 MeV and a total energy of $5\times {10}^{52}\mathrm{erg}$ with a 5 Mton water detector. A range of from 0.7–1.3 on the horizontal axis can be roughly estimated from the high-energy neutrino events observed from SN 1987A (see text).