On the Gamma-Ray Emission of the Andromeda Galaxy M31

Abstract

Using the $\gamma$-ray data obtained with the Large Area Telescope (LAT) onboard {\it the Fermi Gamma-ray Space Telescope (Fermi)} for $\sim$14 years, we examine the high energy emission emanating from the center of the Andromeda Galaxy M31. Different from previously reported results, which show a seemingly extended source, we instead find two individual point sources, one consistent with being at the center and one 0\fdg4 south-east of the center. The emission of the former is well described using a Log-Parabola model, similar to those of previous studies, and that of the latter can be fitted with a power law. We discuss the possible origins for the two sources. M31's central source, now consistent with being a point source, necessitates a revisit of its previously discussed originations with this new property taken into consideration, in particular those cosmic rays or dark matter scenarios involving extended source distributions. The SE source appears to have a projected distance of $\sim$6\,kpc from M31's center, and the investigation is required as to whether it is a source locally associated with M31, or is instead a background extra-galactic one.