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 dark matter or stellar related origins. 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 challenging due to uncertainties in the astrophysical gamma-ray foreground model. 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 data from the PAndAS survey and carefully remove the foreground stars. We also produce improved astrophysical foreground models via novel image inpainting techniques based on machine learning methods. Our stellar maps, mimicking the location of a population of millisecond pulsars in the bulge of M31, reach a 5.4σ significance, making them as strongly favoured as the simple phenomenological models usually considered in the literature, e.g. disk-like templates. This detection 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. We demonstrate that about one million unresolved millisecond pulsars naturally explain the observed gamma-ray luminosity per stellar mass, energy spectrum, and stellar bulge-to-disk flux ratio.

On the Gamma-Ray Emission of the Andromeda Galaxy M31

“…Utilizing the 2 yr data taken with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi), the Fermi-LAT Collaboration (Abdo et al 2010) first reported the detection of M31 at a 5σ confidence level. Following this initial report, different analyses of the LAT data have been performed for studies of the γ-ray emission of M31 (Li et al 2016;Pshirkov et al 2016;Ackermann et al 2017;Karwin et al 2019;Zimmer et al 2022). It appears that the primary γ-ray emission of M31 coincides with its center, and efforts have been made to identify a possible extended structure in the emission, whose presence in M31 is expected and would reveal the existence and distribution of cosmic rays or supposedly massive dark matter.…”

On the Gamma-Ray Emission of the Andromeda Galaxy M31

“…Utillizing the 2-yr data taken with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi), the Fermi-LAT Collaboration (Abdo et al 2010) first reported the detection of M31 at a 5σ confidence level. Following this initial report, different analyses of the LAT data have been performed for studies of the γ-ray emission of M31 (Li et al 2016;Pshirkov et al 2016;Ackermann et al 2017;Karwin et al 2019;Zimmer et al 2022). It appears that the primary γ-ray emission of M31 coincides with its center, and efforts have been made to identify a possible extended structure in the emission, whose presence in M31 is expected and would reveal the existence and distribution of cosmic rays or supposedly massive dark matter.…”

Identifying the Gamma-Ray Emission of the Nearby Galaxy M83

“…However, there are approximately a dozen galaxies, either within the Local Group or nearby, that have been detected at γ-rays (Ajello et al 2020;Xi et al 2020). While there are complications in the production of the γ-ray emissions observed from these galaxies, for example, AGN possibly hiding in some of them (Peng et al 2019) and the γ-ray emission of the Local Group galaxy M31 considered to consist of different components (Li et al 2016;Pshirkov et al 2016;Ackermann et al 2017;Karwin et al 2019;Zimmer et al 2022;Xing et al 2023), γ-ray luminosities of most of them well correlate with infrared (IR) or radio 1.4 GHz luminosities (Abdo et al 2010b;Ackermann et al 2012;Ajello et al 2020;Xi et al 2020). This correlation is considered an indicator of the cosmic-ray (CR) origin of the γ-ray emissions.…”