Abstract: | We report on the first measurement of the astrophysical neutrino flux using
particle showers (cascades) in IceCube data from 2010 -- 2015. Assuming
standard oscillations, the astrophysical neutrinos in this dedicated cascade
sample are dominated ($sim 90 \%$) by electron and tau flavors. The flux,
observed in the energy range from $16,mathrm{TeV} $ to $2.6,mathrm{PeV}$,
is consistent with a single power-law as expected from Fermi-type acceleration
of high energy particles at astrophysical sources. We find the flux spectral
index to be $gamma=2.53pm0.07$ and a flux normalization for each neutrino
flavor of $phi_{astro} = 1.66^{+0.25}_{-0.27}$ at $E_{0} = 100,
mathrm{TeV}$. This flux of electron and tau neutrinos is in agreement with
IceCube muon neutrino results and with all-neutrino flavor results. Results
from fits assuming more complex neutrino flux models suggest a flux softening
at high energies and a flux hardening at low energies (p-value $ge 0.06$). |