XRISM constraints on unidentified X-ray emission lines, including the 3.5 keV line, in the stacked spectrum of ten galaxy clusters

TL;DR

This study uses XRISM's high-resolution X-ray observations of ten galaxy clusters to search for unidentified emission lines, including the debated 3.5 keV line, setting new upper limits on sterile neutrino dark matter decay rates.

Contribution

First to utilize XRISM Resolve data for stacking galaxy cluster spectra to constrain unidentified X-ray lines, including the 3.5 keV line, with improved sensitivity over previous studies.

Findings

No unidentified lines detected in the stacked spectrum.

Upper limit on sterile neutrino decay rate is 3-4 times lower than Hitomi's Perseus result.

Sensitivity may improve with future XRISM observations to test previous detections.

Abstract

We stack 3.75 Megaseconds of early XRISM Resolve observations of ten galaxy clusters to search for unidentified spectral lines in the $E=$ 2.5-15 keV band (rest frame), including the $E=3.5$ keV line reported in earlier, low spectral resolution studies of cluster samples. Such an emission line may originate from the decay of the sterile neutrino, a warm dark matter (DM) candidate. No unidentified lines are detected in our stacked cluster spectrum, with the $3\sigma$ upper limit on the $m_{\rm s}\sim$ 7.1 keV DM particle decay rate (which corresponds to a $E=3.55$ keV emission line) of $\Gamma \sim 1.0 \times 10^{-27}$ s$^{-1}$. This upper limit is 3-4 times lower than the one derived by Hitomi Collaboration et al. (2017) from the Perseus observation, but still 5 times higher than the XMM-Newton detection reported by Bulbul et al. (2014) in the stacked cluster sample. XRISM Resolve, with its high spectral resolution but a small field of view, may reach the sensitivity needed to test the XMM-Newton cluster sample detection by combining several years worth of future cluster observations.