  It is often stated that the observation of high-energy neutrinos from an
astrophysical source would constitute a smoking gun for the acceleration of
hadronic cosmic rays. Here, we point out that there exists a purely leptonic
mechanism to produce TeV-scale neutrinos in astrophysical environments. In
particular, very high-energy synchrotron photons can scatter with X-rays,
exceeding the threshold for muon-antimuon pair production. When these muons
decay, they produce neutrinos without any cosmic-ray protons or nuclei being
involved. In order for this mechanism to be efficient, the source in question
must produce very high-energy photons which interact in an environment that is
dominated by keV-scale radiation. As an example, we consider the active galaxy
NGC 1068, which IceCube has recently detected as a source of TeV-scale
neutrinos. We find that the neutrino emission observed from this source could
potentially be generated through muon pair production for reasonable choices of
physical parameters.
