3.55 keV photon lines from axion to photon conversion in the Milky Way and M31

TL;DR

This paper investigates the hypothesis that the 3.55 keV photon line results from dark matter decay to axion-like particles, which then convert to photons in magnetic fields, with M31 providing promising observational signals.

Contribution

It demonstrates that M31's magnetic environment could produce detectable 3.55 keV signals via ALP-photon conversion, unlike the Milky Way.

Findings

M31's ALP-photon conversion flux is two orders of magnitude higher than the Milky Way's.

Milky Way magnetic field is insufficient for observable signals with current instruments.

The observed 3.55 keV line morphology supports the ALP conversion scenario.

Abstract

We further explore a scenario in which the recently observed 3.55 keV photon line arises from dark matter decay to an axion-like particle (ALP) of energy 3.55 keV, which then converts to a photon in astrophysical magnetic fields. This ALP scenario is well-motivated by the observed morphology of the 3.55 keV flux. For this scenario we study the expected flux from dark matter decay in the galactic halos of both the Milky Way and Andromeda (M31). The Milky Way magnetic field is asymmetric about the galactic centre, and so the resulting 3.55 keV flux morphology differs significantly from the case of direct dark matter decay to photons. However the Milky Way magnetic field is not large enough to generate an observable signal, even with ASTRO-H. In contrast, M31 has optimal conditions for ALP to photon conversion and the intrinsic signal from M31 becomes two orders of magnitude larger than for the Milky Way, comparable to that from clusters and consistent with observations.