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Register a new userProblems and Prospects from a Flood of Extragalactic TeV Neutrinos in IceCube
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Matthew D. Kistler
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The steep spectrum of neutrinos measured by IceCube extending from >1 PeV down to ~10 TeV has an energy flux now encroaching on the Fermi isotropic GeV background. We examine several implications starting from source energetics requirements for neutrino production. We show how the environment of extragalactic nuclei can extinguish ~10-100 TeV gamma rays and convert their energy to X-rays for plausible conditions of infrared luminosity and magnetic field, so that the Fermi background is not overwhelmed by cascades. We address a variety of scenarios, such as for acceleration by supermassive black holes and hadronic scenarios, and observations that may help elucidate the neutrinos shadowy origins.
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Any interpretation of the astrophysical neutrinos discovered by IceCube must accommodate a variety of multimessenger constraints. We address implications of these neutrinos being produced in transient sources, principally if buried within supernovae so that gamma rays are absorbed by the star. This would alleviate tension with the isotropic Fermi GeV background that >10 TeV neutrinos rival in detected energy flux. We find that IceCube data constrain transient properties, implying buried GeV-TeV electromagnetic emission near or exceeding canonical SN explosion energies of ~10^51 erg, indicative of an origin within superluminous SNe. TeV neutrino bursts with dozens of IceCube events -- which would be of great use for understanding r-process nucleosynthesis and more -- may be just around the corner if they are a primary component of the flux.
The IceCube neutrino discovery presents an opportunity to answer long-standing questions in high-energy astrophysics. For their own sake and relations to other processes, it is important to understand neutrinos arising from the Milky Way, which should have an accompanying flux of gamma rays. Examining Fermi TeV data, and applying other constraints up to >1 PeV, it appears implausible that the Galactic fraction of the IceCube flux is large, though could be present at some level. We address Sgr A*, where the TeV-PeV neutrinos may outrun gamma rays due to gamma-gamma opacity, and further implications, including dark matter and cosmic-ray electrons.
The IceCube Neutrino Observatory was designed primarily to search for high-energy (TeV--PeV) neutrinos produced in distant astrophysical objects. A search for $gtrsim 100$~TeV neutrinos interacting inside the instrumented volume has recently provided evidence for an isotropic flux of such neutrinos. At lower energies, IceCube collects large numbers of neutrinos from the weak decays of mesons in cosmic-ray air showers. Here we present the results of a search for neutrino interactions inside IceCubes instrumented volume between 1~TeV and 1~PeV in 641 days of data taken from 2010--2012, lowering the energy threshold for neutrinos from the southern sky below 10 TeV for the first time, far below the threshold of the previous high-energy analysis. Astrophysical neutrinos remain the dominant component in the southern sky down to 10 TeV. From these data we derive new constraints on the diffuse astrophysical neutrino spectrum, $Phi_{ u} = 2.06^{+0.4}_{-0.3} times 10^{-18} left({E_{ u}}/{10^5 ,, rm{GeV}} right)^{-2.46 pm 0.12} {rm {GeV^{-1} , cm^{-2} , sr^{-1} , s^{-1}} } $, as well as the strongest upper limit yet on the flux of neutrinos from charmed-meson decay in the atmosphere, 1.52 times the benchmark theoretical prediction used in previous IceCube results at 90% confidence.
We present two searches for IceCube neutrino events coincident with 28 fast radio bursts (FRBs) and one repeating FRB. The first improves upon a previous IceCube analysis -- searching for spatial and temporal correlation of events with FRBs at energies greater than roughly 50 GeV -- by increasing the effective area by an order of magnitude. The second is a search for temporal correlation of MeV neutrino events with FRBs. No significant correlation is found in either search, therefore, we set upper limits on the time-integrated neutrino flux emitted by FRBs for a range of emission timescales less than one day. These are the first limits on FRB neutrino emission at the MeV scale, and the limits set at higher energies are an order-of-magnitude improvement over those set by any neutrino telescope.
A diffuse flux of astrophysical neutrinos above $100,mathrm{TeV}$ has been observed at the IceCube Neutrino Observatory. Here we extend this analysis to probe the astrophysical flux down to $35,mathrm{TeV}$ and analyze its flavor composition by classifying events as showers or tracks. Taking advantage of lower atmospheric backgrounds for shower-like events, we obtain a shower-biased sample containing 129 showers and 8 tracks collected in three years from 2010 to 2013. We demonstrate consistency with the $(f_e:f_{mu}:f_tau)_oplusapprox(1:1:1)_oplus$ flavor ratio at Earth commonly expected from the averaged oscillations of neutrinos produced by pion decay in distant astrophysical sources. Limits are placed on non-standard flavor compositions that cannot be produced by averaged neutrino oscillations but could arise in exotic physics scenarios. A maximally track-like composition of $(0:1:0)_oplus$ is excluded at $3.3sigma$, and a purely shower-like composition of $(1:0:0)_oplus$ is excluded at $2.3sigma$.
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