Constraints on ultracompact minihalos from the extragalactic gamma-ray
  background observation
  Ultracompact minihalo (UCMH) is a special type of dark matter halo with a
very steep density profile which may form in the early universe seeded by an
overdense region or a primordial black hole. Constraints on its abundance give
valuable information on the power spectrum of primordial perturbation. In this
work, we update the constraints on the UCMH abundance in the universe using the
extragalactic gamma-ray background (EGB) observation. Comparing to previous
works, we adopt the updated Fermi-LAT EGB measurement and derive constraints
based on a full consideration of the astrophysical contributions. With these
improvements, we place constraints on UCMH abundance 1-2 orders of magnitude
better than previous results. With the background components considered, we can
also attempt to search for possible additional components beyond the known
astrophysical contributions.
