  We scrutinize the hypothesis that gauge singlet fermions -- sterile neutrinos
-- interact with Standard Model particles through the transition magnetic
moment portal. These interactions lead to the production of sterile neutrinos
in supernovae followed by their decay into photons and active neutrinos which
can be detected at $\gamma$-ray telescopes and neutrino detectors,
respectively. We find that the non-observation of active neutrinos and photons
from sterile-neutrino decay associated to SN1987A yields the strongest
constraints to date on magnetic-moment-coupled sterile neutrinos if their
masses are inside a $0.1-100$ MeV window. Assuming a near-future galactic
supernova explosion, we estimate the sensitivity of several present and
near-future experiments, including Fermi-LAT, e-ASTROGAM, DUNE, and
Hyper-Kamiokande, to magnetic-moment-coupled sterile neutrinos. We also study
the diffuse photon and neutrino fluxes produced in the decay of magnetic-moment
coupled sterile neutrinos produced in all past supernova explosions and find
that the absence of these decay daughters yields the strongest constraints to
date for sterile neutrino masses inside a $1-100$ keV window.
