Reionization and dark matter decay
  Cosmic reionization and dark matter decay can impact observations of the
cosmic microwave sky in a similar way. A simultaneous study of both effects is
required to constrain unstable dark matter from cosmic microwave background
observations. We compare two reionization models with and without dark matter
decay. We find that a reionization model that fits also data from quasars and
star forming galaxies results in tighter constraints on the reionization
optical depth $\tau_{\text{reio}}$, but weaker constraints on the spectral
index $n_{\text{s}}$ than the conventional parametrization. We use the Planck
2015 data to constrain the effective decay rate of dark matter to $\Gamma_{\rm
eff} < 2.9 \times 10^{-25}/$s at $95$\% C.L. This limit is robust and model
independent. It holds for any type of decaying dark matter and it depends only
weakly on the chosen parametrization of astrophysical reionization. For light
dark matter particles that decay exclusively into electromagnetic components
this implies a limit of $\Gamma < 5.3 \times 10^{-26}/$s at $95$\% C.L.
Specifying the decay channels, we apply our result to the case of keV-mass
sterile neutrinos as dark matter candidates and obtain constraints on their
mixing angle and mass, which are comparable to the ones from the diffuse X-ray
background.
