Glashow resonance as a window into cosmic neutrino sources

The Glashow resonance at E_\nu=6.3 PeV is a measure of the \bar\nu_e content of the astrophysical neutrino flux. The fractional \bar\nu_e content depends on the neutrino production model at the cosmic neutrino source, and the environment at the source. Thus, the strength of the Glashow resonance event rate is a potential window into astrophysical sources. We quantify the "Glashow resonometer" and comment on the significance that no Glashow events are observed in the IceCube three-year data.