Discovery of a 3.5 keV line in the Galactic Center and a Critical Look at the Origin of the Line Across Astronomical Targets

TL;DR

This study critically reanalyzes the 3.5 keV X-ray line claims in various astronomical sources, finding no conclusive evidence for dark matter origin and highlighting the role of plasma lines and systematic uncertainties.

Contribution

It provides a comprehensive reanalysis of the 3.5 keV line across multiple targets, emphasizing the importance of plasma lines and systematic uncertainties in interpreting potential dark matter signals.

Findings

No conclusive excess in the Galactic center, M31, or galaxy clusters.

Known plasma lines can explain the observed features without invoking dark matter.

Detection of a 3.55 keV line in Tycho suggests systematic effects or unusual elemental abundances.

Abstract

We examine the claimed excess X-ray line emission near 3.5 keV including both a new analysis of XMM-Newton observations of the Milky Way center and a reanalysis of the data on M~31 and clusters. In no case do we find conclusive evidence for an excess. In the case of the Galactic center we show that known plasma lines, including in particular K XVIII lines at 3.48 and 3.52 keV, provide a satisfactory fit to the XMM data. We estimate the expected flux of the K XVIII lines and find that the measured line flux falls squarely within the predicted range based on the brightness of other well-measured lines in the energy range of interest and on detailed multi-temperature plasma models. We then re-assess the evidence for excess emission from clusters of galaxies, allowing for systematic uncertainty in the expected flux from known plasma lines and additional uncertainty due to potential variation in the abundances of different elements. We find that no conclusive excess line emission can be advocated when considering systematic uncertainties in Perseus or in other clusters. We also re-analyze the XMM data for M~31 and find no statistically significant line emission near 3.5 keV to a level greater than one sigma. Finally, we analyze the Tycho supernova remnant, which shows similar plasma features to the sources above, but does not host any significant dark matter. We detect a 3.55 keV line from Tycho, which points to possible systematic effects in the flux determination of weak lines, or to relative elemental abundances vastly different from theoretical expectations.