Dark matter subhalos and unidentified sources in the Fermi 3FGL source
  catalog
  If dark matter consists of weakly interacting massive particles (WIMPs), dark
matter subhalos in the Milky Way could be detectable as gamma-ray point sources
due to WIMP annihilation. In this work, we perform an updated study of the
detectability of dark matter subhalos as gamma-ray sources with the Fermi Large
Area Telescope (Fermi LAT). We use the results of the Via Lactea II simulation,
scaled to the Planck 2015 cosmological parameters, to predict the local dark
matter subhalo distribution. Under optimistic assumptions for the WIMP
parameters --- a 40 GeV particle annihilating to $b\bar{b}$ with a thermal
cross-section, as required to explain the Galactic center GeV excess --- we
predict that at most $\sim 10$ subhalos might be present in the third Fermi LAT
source catalog (3FGL). This is a smaller number than has been predicted by
prior studies, and we discuss the origin of this difference. We also compare
our predictions for the detectability of subhalos with the number of subhalo
candidate sources in 3FGL, and derive upper limits on the WIMP annihilation
cross-section as a function of the particle mass. If a dark matter
interpretation could be excluded for all 3FGL sources, our constraints would be
competitive with those found by indirect searches using other targets, such as
known Milky Way satellite galaxies.
