Estimates for a DM $\rightarrow a \rightarrow \gamma$ 3.55 keV line in the radio lobes of Centaurus A

Abstract

The origin of the 3.55 keV excess reported in various galaxies and galaxy clusters remains unknown. Non-observations of the line in dwarf spheroidal galaxies (dSphs) create tension with galaxy cluster observations when considering a direct Dark Matter (DM) $\rightarrow \gamma$ decay. This discrepancy could be explained by a DM $\rightarrow a \rightarrow \gamma$ decay, where the DM particles decay to axion-like particles (ALPs), which then convert to photons in a magnetic field. The 3.55 keV line from a decay to ALPs therefore has very different morphology from a direct decay to photons, as it would only appear in environments with substantial magnetic fields between the source and us. To test this hypothesis we consider targets where the line strength would be enhanced compared to the DM $\rightarrow \gamma$ model. The giant lobes of radio galaxies could represent good environments, having magnetic fields with strength $\mathcal{O}(\mu\rm{G})$ that extend for hundreds of kiloparsecs, and minimal X-ray background. In the case of Centaurus A, a large population of dSphs are associated with the galaxy, some of which are potentially in or behind its lobes. In this paper we estimate the flux of a 3.55 keV DM $\rightarrow a \rightarrow \gamma$ line from these sources, and compare it to the flux from the DM halo of Centaurus~A. We comment on the potential to detect such a signal with current satellites (such as XMM-Newton and Chandra) and future satellites (such as Athena).