  The flux, spectrum and angular distribution of the excess neutrino signal
detected by IceCube between 50TeV and 2PeV are inconsistent with those expected
for Galactic sources. The coincidence of the excess,
$E_\nu^2\Phi_\nu=3.6\pm1.2\times10^{-8}(GeV/ cm^2 sr s)$, with the
Waxman-Bahcall (WB) bound, $E_\nu^2\Phi_{WB}=3.4\times10^{-8}(GeV/cm^2 sr s)$,
is probably a clue to the origin of IceCube's neutrinos. The most natural
explanation of this coincidence is that both the neutrino excess and the
ultra-high energy, $>10^{19}$ eV, cosmic-ray (UHECR) flux are produced by the
same population of cosmologically distributed sources, producing CRs, likely
protons, at a similar rate, $E^2 dQ/dE=0.5\times10^{44}(erg/Mpc^3yr)$ (at z=0),
across a wide range of energies, from $10^{15}$ eV to $>10^{20}$ eV, and
residing in environments (such as starburst galaxies) in which CRs of rigidity
$E/Z< 10^{17}$ eV lose much of their energy to pion production. Identification
of the neutrino sources will allow one to identify the UHECR accelerators, to
resolve open questions related to the accelerator models, and to study neutrino
properties (related e.g. to flavor oscillations and coupling to gravity) with
an accuracy many orders of magnitude better than is currently possible. The
most promising method for identifying the sources is by association of a
neutrino with an electromagnetic signal accompanying a transient event
responsible for its generation. The neutrino flux that is produced within the
sources, and that may thus be directly associated with transient events, may be
significantly lower than the total observed neutrino flux, which may be
dominated by neutrino production at the environment in which the sources
reside.
