  We infer the ultrahigh energy neutrino source by using the Glashow resonance
candidate event recently identified by the IceCube Observatory. For the
calculation of the cross section for the Glashow resonance, we incorporate both
the atomic Doppler broadening effect and initial state radiation
$\overline{\nu}^{}_{e} e^- \to W^- \gamma$, which correct the original cross
section considerably. Using available experimental information, we have set a
generic constraint on the $\overline{\nu}^{}_{e}$ fraction of astrophysical
neutrinos, which excludes the $\mu$-damped ${\rm p}\gamma$ source around
$2\sigma$ confidence level under the assumption that neutrino production is
dominated by the $\Delta$-resonance. While a weak preference has been found for
the pp source, next-generation measurements will be able to distinguish between
ideal pp and p$\gamma$ sources with a high significance assuming an optimistic
single power-law neutrino spectrum. The inclusion of multi-pion production at
very high energies for the neutrino source can weaken the discrimination power.
In this case additional multimessenger information is needed to distinguish
between pp and p$\gamma$ sources.
