The Supernova Early Warning System

A core collapse in the Milky Way will produce an enormous burst of neutrinos in detectors worldwide. Such a burst has the potential to provide an early warning of a supernova's appearance. I will describe the nature of the signal, the sensitivity of current detectors, and SNEWS, the SuperNova Early Warning System, a network designed to alert astronomers as soon as possible after the detected neutrino signal. 1 The supernova neutrino signal When a massive star reaches the end of its life, its core collapses. More than 99% of the binding energy of the resulting neutron star is released in the form of neutrinos and antineu-trinos of all flavors, with energies in the tens of MeV range: the energy leaves via neutrinos because neutrinos interact so weakly that they readily leave the star. The neutrinos themselves bring information from deep inside the core; the detection of such a signal will yield great insights into neutrino properties and core collapse physics (e.g. Raffelt 2007). Of particular interest for this workshop is that the timescale of neutrino emission is a few tens of seconds, promptly after core collapse; the photon signal, in contrast, can take hours or even days to emerge from the stellar envelope. Thus, the detection of a burst of neutrinos will give astronomers an early warning of a nearby supernova. So far there has been only one observed instance of core collapse neutrino emission: for SN1987A in the LMC, two water Cherenkov detectors, IMB and Kamiokande II, observed a total of 19 neutrino interactions between them (Hi-rata et al. 1987; Bionta et al. 1987). Two scintillator detector observations were also reported (Alekseev et al. 1987; Aglietta et al. 1987). The water Cherenkov neutrinos were recorded 2.5 hours before the first light was observed from the supernova; however the neutrino signal was only retrieved from data tapes after the fact. Next time we will do better. An early observation of the supernova light curve turn-on will bring information about the progenitor and its environment, which can in turn feed back to neutrino physics. The aim of SNEWS is to provide the astronomical community with an early warning of a supernova's occurrence, as well as to improve global sensitivity to a supernova neutrino burst via inter-experiment collaboration. 2 Neutrino detectors Because neutrinos interact so weakly, in spite of their huge flux, enormous detectors are required to see a substantial signal. Typically …