Millisecond pulsars and the gamma-ray excess in Andromeda
  The Fermi Gamma-Ray Space Telescope has provided evidence for diffuse
gamma-ray emission in the central parts of the Milky Way and the Andromeda
galaxy. This excess has been interpreted either as dark matter annihilation
emission or as emission from thousands of millisecond pulsars (MSPs). We have
recently shown that old massive globular clusters may move towards the center
of the Galaxy by dynamical friction and carry within them enough MSPs to
account for the observed gamma-ray excess. In this paper we revisit the MSP
scenario for the Andromeda galaxy, by modeling the formation and disruption of
its globular cluster system. We find that our model predicts gamma-ray emission
$\sim 2-3$ times larger than for the Milky Way, but still nearly an order of
magnitude smaller than the observed Fermi excess in the Andromeda. Our MSP
model can reproduce the observed excess only by assuming $\sim 8$ times larger
number of old clusters than inferred from galaxy scaling relations. To explain
the observations we require either that Andromeda deviates significantly from
the scaling relations, or that a large part of its high-energy emission comes
from additional sources.
