Was There a 3.5 keV Line?

TL;DR

This paper critically re-examines the evidence for the 3.5 keV emission line, finding little robust support for its existence after reanalysis and highlighting issues with previous claims and data modeling.

Contribution

The study systematically reproduces previous analyses and demonstrates that most lack significant evidence for the 3.5 keV line, emphasizing the importance of robust data modeling and analysis methods.

Findings

Most previous detections of the 3.5 keV line are not reproducible.

Background mismodeling affects the detection claims.

Narrow energy window analysis reduces modeling issues without confirming the line.

Abstract

The 3.5 keV line is a purported emission line observed in galaxies, galaxy clusters, and the Milky Way whose origin is inconsistent with known atomic transitions and has previously been suggested to arise from dark matter decay. We systematically re-examine the bulk of the evidence for the 3.5 keV line, attempting to reproduce six previous analyses that found evidence for the line. Surprisingly, we only reproduce one of the analyses; in the other five we find no significant evidence for a 3.5 keV line when following the described analysis procedures on the original data sets. For example, previous results claimed 4$\sigma$ evidence for a 3.5 keV line from the Perseus cluster; we dispute this claim, finding no evidence for a 3.5 keV line. We find evidence for background mismodeling in multiple analyses. We show that analyzing these data in narrower energy windows diminishes the effects of mismodeling but returns no evidence for a 3.5 keV line. We conclude that there is little robust evidence for the existence of the 3.5 keV line. Some of the discrepancy of our results from those of the original works may be due to the earlier reliance on local optimizers, which we demonstrate can lead to incorrect results. For ease of reproducibility, all code and data are publicly available.