  We provide a comprehensive study of observable spectra from dark matter
pair-annihilation or decay into sterile (right-handed) neutrinos. This occurs,
for instance, in neutrino portal dark matter models, where a sterile neutrino
acts as the portal between dark matter and the Standard Model sector. The
subsequent decays of right-handed neutrinos produce detectable Standard Model
particles, notably photons, positrons, and neutrinos. We study the
phenomenology of models where the right-handed neutrino masses are below the
GeV scale, as well as models where they are at, or significantly heavier than,
the TeV scale. In both instances, and for different reasons, the standard
tools, including Monte Carlo simulations, are both inadequate and inaccurate.
We present the complete framework to compute the relevant branching ratios for
right-handed neutrino decays and the spectra of secondary photons, positrons,
and neutrinos for a broad range of dark matter and right-handed neutrino
masses. We discuss the general features of such signals, and compare the
spectra to standard signals from dark matter annihilation/decay into bottom
quarks. Additionally, we provide open source code1 that can be used to compute
such spectra. The code is available at
https://github.com/LoganAMorrison/blackthorn.
