Kavli Affiliate: Shunsaku Horiuchi
| First 5 Authors: Fabian Zimmer, Oscar Macias, Shin’ichiro Ando, Roland M. Crocker, Shunsaku Horiuchi
| Summary:
Since the discovery of an excess in gamma rays in the direction of M31, its
cause has been unclear. Published interpretations focus on a dark matter or
stellar related origin. Studies of a similar excess in the Milky Way center
motivate a correlation of the spatial morphology of the signal with the
distribution of stellar mass in M31. However, a robust determination of the
best theory for the observed excess emission is very challenging due to large
uncertainties in the astrophysical gamma-ray foreground model. Here we perform
a spectro-morphological analysis of the M31 gamma-ray excess using
state-of-the-art templates for the distribution of stellar mass in M31 and
novel astrophysical foreground models for its sky region. We construct maps for
the old stellar populations of M31 based on observational data from the PAndAS
survey and carefully remove the foreground stars. We also produce improved
astrophysical foreground models by using novel image inpainting techniques
based on machine learning methods. We find that our stellar maps, taken as a
proxy for the location of a putative population of millisecond pulsars in the
bulge of M31, reach a statistical significance of $5.4sigma$, making them as
strongly favoured as the simple phenomenological models usually considered in
the literature, e.g., a disk-like template with uniform brightness. Our
detection of the stellar templates is robust to generous variations of the
astrophysical foreground model. Once the stellar templates are included in the
astrophysical model, we show that the dark matter annihilation interpretation
of the signal is unwarranted. Using the results of a binary population
synthesis model we demonstrate that a population of about one million
unresolved MSPs could naturally explain the observed gamma-ray luminosity per
stellar mass, energy spectrum, and stellar bulge-to-disk flux ratio.
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