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Since the discovery of an excess in gamma rays in the direction of M31, itscause has been unclear. Published interpretations focus on a dark matter orstellar related origin. Studies of a similar excess in the Milky Way centermotivate a correlation of the spatial morphology of the signal with thedistribution of stellar mass in M31. However, a robust determination of thebest theory for the observed excess emission is very challenging due to largeuncertainties in the astrophysical gamma-ray foreground model. Here we performa spectro-morphological analysis of the M31 gamma-ray excess usingstate-of-the-art templates for the distribution of stellar mass in M31 andnovel astrophysical foreground models for its sky region. We construct maps forthe old stellar populations of M31 based on observational data from the PAndASsurvey and carefully remove the foreground stars. We also produce improvedastrophysical foreground models by using novel image inpainting techniquesbased on machine learning methods. We find that our stellar maps, taken as aproxy for the location of a putative population of millisecond pulsars in thebulge of M31, reach a statistical significance of $5.4\sigma$, making them asstrongly favoured as the simple phenomenological models usually considered inthe literature, e.g., a disk-like template with uniform brightness. Ourdetection of the stellar templates is robust to generous variations of theastrophysical foreground model. Once the stellar templates are included in theastrophysical model, we show that the dark matter annihilation interpretationof the signal is unwarranted. Using the results of a binary populationsynthesis model we demonstrate that a population of about one millionunresolved MSPs could naturally explain the observed gamma-ray luminosity perstellar mass, energy spectrum, and stellar bulge-to-disk flux ratio.

The Andromeda Gamma-Ray Excess: Background Systematics of the Millisecond Pulsars and Dark Matter Interpretations