Purity of the Real-Time Neutrino Alert Stream: Expected True and False Alerts Per Year

Since 2016, IceCube has issued public real-time alerts for individual high-energy neutrinos likely to be astrophysical, graded Gold and Bronze by their estimated probability of astrophysical origin, called signalness. This paper converts the published operational rates and signalness values into expected counts. Gold alerts, at 12 per year with signalness one half, yield 6 true astrophysical and 6 false alerts annually. Bronze alerts, at 20 per year with signalness 0.3, yield 6 true and 14 false. The combined stream of 32 alerts per year carries 12 true events and an overall purity of 0.375. At two hours of follow-up observing per alert, the community spends 64 hours a year chasing the stream, more than half of it on atmospheric background. The probability that at least one Gold alert in a year is genuinely astrophysical is 1 — the stream is noisy per event and reliable in aggregate, which is exactly the regime its users must plan for. The study is computed from stated assumptions, not measured.