Consistency of dark matter interpretations of the 3.5 keV X-ray line

TL;DR

This paper explores whether the 3.5 keV X-ray line observed in some astrophysical systems can be explained by excited dark matter models, which predict different signals depending on the system's velocity dispersion.

Contribution

It introduces the excited dark matter (XDM) mechanism as an alternative explanation for the 3.5 keV line, reconciling positive and negative observational results.

Findings

XDM predicts stronger signals in high-velocity systems like galaxy clusters.

Null results in dwarf satellites are consistent with XDM.

Close galaxies could produce detectable signals matching cluster fluxes.

Abstract

Tentative evidence of a 3.5 keV X-ray line has been found in the stacked spectra of galaxy clusters, individual clusters, the Andromeda galaxy and the galactic center, leading to speculation that it could be due to decays of metastable dark matter such as sterile neutrinos. However searches for the line in other systems such as dwarf satellites of the Milky Way have given negative or ambiguous results. We reanalyze both the positive and negative searches from the point of view that the line is due to inelastic scattering of dark matter to an excited state that subsequently decays---the mechanism of excited dark matter (XDM). Unlike the metastable dark matter scenario, XDM gives a stronger signal in systems with higher velocity dispersions, such as galaxy clusters. We show that the predictions of XDM can be consistent with null searches from dwarf satellites, while the signal from the closest individual galaxies can be detectable having a flux consistent with that from clusters. We discuss the impact of our new fits to the data for two specific realizations of XDM.