  We discuss how to constrain new physics in the neutrino sector using
multimessenger astronomical observations by the IceCube experiment. The
information from time and direction coincidence with an identifiable source is
used to improve experimental limits by constraining the mean free path of
neutrinos from these sources. Over the coming years, IceCube is expected to
detect neutrinos from a variety of neutrino-producing sources, and has already
identified the Blazar TXS 0506+056 as a neutrino-producing source. We explore
specific phenomenological models: additional neutrino interactions,
neutrinophilic dark matter, and lepton-number-charged axion dark matter. For
each new physics scenario, we interpret the observation of neutrinos from TXS
0506+056 as a constraint on the parameters of the new physics models. We also
discuss mergers involving neutron stars and black holes, and how the detection
of neutrinos coincident with these events could place bounds on the new physics
models.
