| Abstract: | Evidence for an extraterrestrial flux of high-energy neutrinos has now been
found in multiple searches with the IceCube detector. The first solid evidence
was provided by a search for neutrino events with deposited energies
$gtrsim30$~TeV and interaction vertices inside the instrumented volume. Recent
analyses suggest that the extraterrestrial flux extends to lower energies and
is also visible with throughgoing, $
u_mu$-induced tracks from the Northern
hemisphere. Here, we combine the results from six different IceCube searches
for astrophysical neutrinos in a maximum-likelihood analysis. The combined
event sample features high-statistics samples of shower-like and track-like
events. The data are fit in up to three observables: energy, zenith angle and
event topology. Assuming the astrophysical neutrino flux to be isotropic and to
consist of equal flavors at Earth, the all-flavor spectrum with neutrino
energies between 25~TeV and 2.8~PeV is well described by an unbroken power law
with best-fit spectral index $-2.50pm0.09$ and a flux at 100~TeV of
$left(6.7_{-1.2}^{+1.1}
ight)cdot10^{-18},mathrm{GeV}^{-1}mathrm{s}^{-1}mathrm{sr}^{-1}mathrm{cm}^{-2}$.
Under the same assumptions, an unbroken power law with index $-2$ is disfavored
with a significance of 3.8~$sigma$ ($p=0.0066\%$) with respect to the best
fit. This significance is reduced to 2.1~$sigma$ ($p=1.7\%$) if instead we
compare the best fit to a spectrum with index $-2$ that has an exponential
cut-off at high energies. Allowing the electron neutrino flux to deviate from
the other two flavors, we find a $
u_e$~fraction of $0.18pm0.11$ at Earth.
The sole production of electron neutrinos, which would be characteristic of
neutron-decay dominated sources, is rejected with a significance of
3.6~$sigma$ ($p=0.014\%$). |
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