Millisecond pulsars and the gamma-ray excess in Andromeda

TL;DR

This study models millisecond pulsars in Andromeda's globular clusters to explain the gamma-ray excess, finding the MSP scenario alone is insufficient without additional sources or deviations from scaling relations.

Contribution

It revisits the MSP hypothesis for Andromeda's gamma-ray excess by modeling globular cluster formation and disruption, highlighting the need for more clusters or other sources.

Findings

Model predicts gamma-ray emission 2-3 times higher than Milky Way.

Predicted emission is nearly an order of magnitude below observed excess.

Reproducing the excess requires about 8 times more old clusters than expected.

Abstract

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.