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We investigate the capability of the Probe Of Extreme Multi-Messenger Astrophysics (POEMMA) in performing target-of-opportunity (ToO) neutrino observations. POEMMA is a proposed space-based probe-class mission for ultrahigh-energy cosmic ray and very-high-energy neutrino detection using two spacecraft, each equipped with a large Schmidt telescope to detect optical and near-ultraviolet signals generated by extensive air showers (EASs). POEMMA will be sensitive to Cherenkov radiation from upward-moving EASs initiated by tau neutrinos interacting in the Earth. POEMMA will be able to quickly repoint (90° in 500 s) each of the two spacecrafts to the direction of an astrophysical source, which in combination with its orbital speed will provide it with unparalleled capability to follow-up transient alerts. We calculate POEMMA's transient sensitivity for two observational configurations for the satellites (ToO-stereo and ToO-dual for smaller and larger satellite separations, respectively) and investigate the impact of variations arising due to POEMMA's orbital characteristics on its sensitivity to tau neutrinos in various regions of the sky. We explore separate scenarios for long (~105-6 s) and short (~10³ s) duration events, accounting for intrusion from the Sun and the Moon in the long-duration scenario. We compare the sensitivity and sky coverage of POEMMA for ToO observations with those for existing experiments (e.g., IceCube, ANTARES, and the Pierre Auger Observatory) and other proposed future experiments (e.g., GRAND200k). For long bursts, we find that POEMMA will provide a factor of ≳7 improvement in average neutrino sensitivity above 300 PeV with respect to existing experiments, reaching the level of model predictions for neutrino fluences at these energies and above from several types of long-duration astrophysical transients (e.g., binary neutron star mergers and tidal disruption events). For short bursts, POEMMA will improve the sensitivity over existing experiments by at least an order of magnitude for Eν≳100 PeV in the "best-case" scenario. POEMMA's orbital characteristics and rapid repointing capability will provide it access to the full celestial sky, including regions that will not be accessible to ground-based neutrino experiments. Finally, we discuss the prospects for POEMMA to detect neutrinos from candidate astrophysical neutrino sources in the nearby Universe. Our results demonstrate that with its improved neutrino sensitivity at ultrahigh energies and unique full-sky coverage, POEMMA will be an essential, complementary component in a rapidly expanding multimessenger network. [ABSTRACT FROM AUTHOR]

The Probe of Extreme Multimessenger Astrophysics (POEMMA) is a potential NASA Astrophysics Probe-class mission designed to observe ultrahigh-energy cosmic rays (UHECRs) and cosmic neutrinos from space. POEMMA will monitor colossal volumes of the Earth's atmosphere to detect extensive air showers (EASs) produced by extremely energetic cosmic messengers: UHECRs above 20 EeV over the full sky and cosmic neutrinos above 20 PeV. We focus most of this study on the impact of POEMMA for UHECR science by simulating the detector response and mission performance for EAS from UHECRs. We show that POEMMA will provide a significant increase in the statistics of observed UHECRs at the highest energies over the entire sky. POEMMA will be the first UHECR fluorescence detector deployed in space that will provide high-quality stereoscopic observations of the longitudinal development of air showers. Therefore it will be able to provide event-by-event estimates of the calorimetric energy and nuclear mass of UHECRs. The particle physics in the interactions limits the interpretation of the shower maximum on an event-by-event basis. In contrast, the calorimetric energy measurement is significantly less sensitive to the different possible final states in the early interactions. POEMMA will increase by a factor of 30 fluorescence observations, with accurate measurements of the shower maximum. We study the prospects to discover the origin and nature of UHECRs using expectations for measurements of the energy spectrum, the distribution of arrival direction, and the atmospheric column depth at which the EAS longitudinal development reaches maximum. We also explore supplementary science capabilities of POEMMA through its sensitivity to particle interactions at extreme energies and its ability to detect ultrahigh-energy neutrinos and photons produced by top-down models including cosmic strings and superheavy dark matter particle decay in the halo of the Milky Way. [ABSTRACT FROM AUTHOR]

The sources of ultrahigh-energy cosmic rays (UHECRs) have been difficult to catch. It was recently pointed out that while sources of UHECR protons exhibit anisotropy patterns that become denser and compressed with rising energy, nucleus-emitting sources give rise to a cepa stratis (onionlike) structure with layers that become more distant from the source position with rising energy. The peculiar shape of the hot spots from nucleus accelerators is steered by the competition between energy loss during propagation and deflection on the Galactic magnetic field (GMF). Here, we run a full-blown simulation study to accurately characterize the deflections of UHECR nuclei in the GMF. We show that while the cepa stratis structure provides a global description of anisotropy patterns produced by UHECR nuclei en route to Earth, the hot spots are elongated depending on their location in the sky due to the regular structure of the GMF. We demonstrate that with a high-statistics sample at the high-energy end of the spectrum, like the one to be collected by NASA's Probe Of Extreme Multi-Messenger Astrophysics mission, the energy dependence of the hot spot contours could become a useful observable to identify the nuclear composition of UHECRs. This new method to determine the nature of the particle species is complementary to those using observables of extensive air showers and therefore is unaffected by the large systematic uncertainties of hadronic interaction models. [ABSTRACT FROM AUTHOR]