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      Neutrino Flavor Evolution in Neutron Star Mergers

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          Abstract

          We examine the flavor evolution of neutrinos emitted from the disk-like remnant (hereafter called \lq\lq neutrino disk\rq\rq) of a binary neutron star (BNS) merger. We specifically look at the neutrinos emitted from the center of the disk, along the polar axis perpendicular to the equatorial plane. In order to better understand the underlying physics and the nature of the ensuing flavor evolution, we carried out two-flavor simulations using a variety of different possible initial neutrino luminosities and energy spectra, and for comparison, also a three-flavor simulation for one of the cases (the bipolar spectral swap). The flavor evolution was found to be highly dependent on the initial neutrino luminosities and energy spectra; in particular, we found two broad classes of results depending on the sign of the initial net electron neutrino lepton number (i.e., the number of neutrinos minus the number of antineutrinos). In the antineutrino dominated case, we found that the Matter-Neutrino Resonance (MNR) effect dominates, consistent with previous literature, whereas in the neutrino dominated case, a bipolar spectral swap develops. For the latter case, in addition to the swap at low energies, a particularly interesting feature of our results was the development of a high energy electron neutrino tail. We argue that the high energy electron neutrinos in the tail of the distribution may have implications for the electron fraction of the material they interact with, and could thereby influence the \(r\)-process in BNS merger environments.

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          Oscillating neutrinos in the early Universe

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            Neutrino oscillations in the early universe

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              Neutrino-driven winds from neutron star merger remnants

              We present a detailed, 3D hydrodynamics study of the neutrino-driven winds that emerge from the remnant of a NS merger. Our simulations are performed with the Newtonian, Eulerian code FISH, augmented by a detailed, spectral neutrino leakage scheme that accounts for heating due to neutrino absorption in optically thin conditions. Consistent with the 2D study of Dessart et al. (2009), we find that a strong baryonic wind is blown out along the original binary rotation axis within \(100\) ms after the merger. We compute a lower limit on the expelled mass of \(3.5 \times 10^{-3} M_{\odot}\), large enough to be relevant for heavy element nucleosynthesis. The physical properties vary significantly between different wind regions. For example, due to stronger neutrino irradiation, the polar regions show substantially larger \(Y_e\) than those at lower latitudes. This has its bearings on the nucleosynthesis: the polar ejecta produce interesting r-process contributions from \(A\sim 80\) to about 130, while the more neutron-rich, lower-latitude parts produce also elements up to the third r-process peak near \(A\sim 195\). We also calculate the properties of electromagnetic transients that are powered by the radioactivity in the wind, in addition to the macronova transient that stems from the dynamic ejecta. The high-latitude (polar) regions produce UV/optical transients reaching luminosities up to \(10^{41} {\rm erg \, s^{-1}}\), which peak around 1 day in optical and 0.3 days in bolometric luminosity. The lower-latitude regions, due to their contamination with high-opacity heavy elements, produce dimmer and more red signals, peaking after \(\sim 2\) days in optical and infrared. Our numerical experiments indicate that it will be difficult to infer the collapse time-scale of the HMNS to a BH based on the wind electromagnetic transient, at least for collapse time-scales larger than the wind production time-scale.
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                Author and article information

                Journal
                2017-03-08
                Article
                1703.03039
                2dd74930-06b9-47c9-8c07-f14d204b2eeb

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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                Custom metadata
                19 pages, 14 figures, for movies see Ancillary files
                astro-ph.HE

                High energy astrophysical phenomena
                High energy astrophysical phenomena

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