During the Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) mission, we planned Target of Opportunity (ToO) operations to follow up on possible sources of ≳ 10 PeV neutrinos. The original plan before flight was to point the onboard Cherenkov Telescope (CT) to catch the source's path on the sky just below Earth's horizon. By using the Earth as a tau-neutrino to tau-lepton converter, the CT would then be able to look for optical extensive air shower signals induced by tau-lepton decays in the atmosphere. The CT had a field of view of 6.4 • vertical × 12.8 • horizontal. Possible neutrino source candidates include gamma ray bursts, tidal disruption events and other bursting or flaring sources. In addition, follow-ups of binary neutron star mergers would have been possible after the start of the O4 observation run from LIGO-Virgo-KAGRA. The resulting exposure is modeled using the NuSpaceSim framework in ToO mode. With the launch of the EUSO-SPB2 payload on the 13th May 2023, this summarizes the ToO program status and preliminary data, as available.

Ultra-high-energy neutrinos serve as messengers of some of the highest energy astrophysical environments. Given that neutrinos are neutral and only interact via weak interactions, neutrinos can emerge from sources, traverse astronomical distances, and point back to their origins. Their weak interactions require large target volumes for neutrino detection. Using the Earth as a neutrino converter, terrestrial, sub-orbital, and satellite-based instruments are able to detect signals of neutrino-induced extensive air showers. In this paper, we describe the software code nuPyProp that simulates tau neutrino and muon neutrino interactions in the Earth and predicts the spectrum of the τ-leptons and muons that emerge. The nuPyProp outputs are lookup tables of charged lepton exit probabilities and energies that can be used directly or as inputs to the nuSpaceSim code designed to simulate optical and radio signals from extensive air showers induced by the emerging charged leptons. We describe the inputs to the code, demonstrate its flexibility and show selected results for τ-lepton and muon exit probabilities and energy distributions. The nuPyProp code is open source, available on github.

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