Observational consistency and future predictions for a 3.5 keV ALP to photon line

TL;DR

This paper investigates whether axion-like particle to photon conversion near the galactic center can explain the 3.5 keV X-ray line, emphasizing the role of magnetic field strength and predicting observational signatures.

Contribution

It demonstrates that a strong magnetic field in the galactic center can produce the 3.5 keV line via ALP-photon conversion, providing a potential dark matter explanation.

Findings

High magnetic fields (>1 mG) can generate observable signals.

The line predominantly originates from regions with z > 20 pc.

The model explains the non-observation in stacked galaxy spectra.

Abstract

Motivated by the possibility of explaining the 3.5 keV line through dark matter decaying to axion-like particles that subsequently convert to photons, we study ALP-photon conversion for sightlines passing within 50 pc of the galactic centre. Conversion depends on the galactic centre magnetic field which is highly uncertain. For fields at low or mid-range of observational estimates (10--100 $\mu$G), no observable signal is possible. For fields at the high range of observational estimates (a pervasive poloidal mG field over the central 150 pc) it is possible to generate sufficient signal to explain recent observations of a 3.5 keV line in the galactic centre. In this scenario, the galactic centre line signal comes predominantly from the region with $z > 20$ pc, reconciling the results from the Chandra and XMM-Newton X-ray telescopes. The dark matter to ALP to photon scenario also naturally predicts the non-observation of the 3.5 keV line in stacked galaxy spectra. We further explore predictions for the line flux in galaxies and suggest a set of galaxies that is optimised for observing the 3.5 keV line in this model.