# Exploring A Cosmic-Ray Origin of the Multi-wavelength Emission in M31

## TL;DR

This paper investigates whether cosmic rays can explain the multi-wavelength emission observed in M31, proposing a multi-component model that fits the data but suggests higher cosmic-ray power than typical sources.

## Contribution

It introduces a comprehensive cosmic-ray based model for M31's emission, considering primary electrons, secondary particles from protons, and their combined effects.

## Key findings

- Multi-component cosmic-ray model fits M31's spectrum.
- Model requires higher CR power than typical supernovae.
- Both primary and secondary CR processes are relevant.

## Abstract

A recent detection of spatially extended gamma-ray emission in the central region of the Andromeda galaxy (M31) has led to several possible explanations being put forth, including dark matter annihilation and millisecond pulsars. Another possibility is that the emission in M31 can be accounted for with a purely astrophysical cosmic-ray (CR) scenario. This scenario would lead to a rich multi-wavelength emission that can, in turn, be used to test it. Relativistic cosmic-ray electrons (CRe) in magnetic fields produce radio emission through synchrotron radiation, while X-rays and gamma rays are produced through inverse Compton scattering. Additionally, collisions of primary cosmic-ray protons (CRp) in the interstellar medium produce charged and neutral pions that then decay into secondary CRe (detectable through radiative processes) and gamma-rays. Here, we explore the viability of a CR origin for multi-wavelength emission in M31, taking into consideration three scenarios: a CR scenario dominated by primary CRe, one dominated by CRp and the resulting secondary CRe and gamma rays from neutral pion decay, and a final case in which both of these components exist simultaneously. We find that the multi-component model is the most promising, and is able to fit the multi-wavelength spectrum for a variety of astrophysical parameters consistent with previous studies of M31 and of cosmic-ray physics. However, the CR power injection implied by our models exceeds the estimated CR power injection from typical astrophysical cosmic-ray sources such as supernovae.