Decaying Dark Matter in Halos of Primordial Black Holes
  We investigate photon signatures of general decaying dark-matter particles in
halos of primordial black holes. We derive the halo-profile density and the
total decay rate for these combined dark-matter scenarios. For the case of
axion-like particles of masses below $\mathcal{O}( 1 )\,$keV, we find strong
bounds on the decay constant which are several orders of magnitude stronger
than the strongest existing bounds, for all halo masses above $\mathcal{O}(
10^{-5} )$ solar masses. Using future X-ray measurements, it will be possible
to push these bounds on such combined dark-matter scenarios even further.
