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

Abstract

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 $5.4\sigma$, 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.

15 pages, 11 figures, comments are welcome

References in corpus (22)

• The Dartmouth Stellar Evolution Database
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV
• The Parkes multibeam pulsar survey: VI. Discovery and timing of 142 pulsars and a Galactic population analysis
• A Tale of Tails: Dark Matter Interpretations of the Fermi GeV Excess in Light of Background Model Systematics
• The long bar as seen by the VVV survey: I. Colour-magnitude diagrams
• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• Millisecond pulsars and the Galactic Center gamma-ray excess: the importance of luminosity function and secondary emission
• Observations of M31 and M33 with the Fermi Large Area Telescope: a galactic center excess in Andromeda?
• The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum
• Balancing mass and momentum in the Local Group
• Millisecond Pulsars from Accretion Induced Collapse as the Origin of the Galactic Centre Gamma-ray Excess Signal
• Search for $\gamma$-ray emission from dark matter particle interactions from Andromeda and Triangulum Galaxies with the Fermi Large Area Telescope
• Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
• The magnetic field structure of the central region in M31
• Cherenkov Telescope Array sensitivity to the putative millisecond pulsar population responsible for the Galactic center excess
• Inverse Compton emission from millisecond pulsars in the Galactic bulge
• Gamma-ray image reconstruction of the Andromeda galaxy
• A Multi-frequency analysis of possible Dark Matter Contributions to M31 Gamma-Ray Emissions
• Analysing the radio flux density profile of the M31 galaxy: a possible dark matter interpretation
• Fermi-LAT Observations of Gamma-Ray Emission Towards the Outer Halo of M31
• Assessing the Impact of Hydrogen Absorption on the Characteristics of the Galactic Center Excess

Cited by in corpus (5)

• TeV Dark Matter Searches in the Extragalactic Gamma-Ray Sky
• On the Gamma-Ray Emission of the Andromeda Galaxy M31
• Identifying the Gamma-ray Emission of the Nearby Galaxy M83
• CONGRuENTS (COsmic-ray, Neutrino, Gamma-ray and Radio Non-Thermal Spectra). II. Population-level correlations between galactic infrared, radio, and γ-ray emission
• Nature of M31 gamma-ray halo in relation to dark matter annihilation