<i>Hitomi</i> Constraints on the 3.5 keV Line in the Perseus Galaxy Cluster

Abstract

<p>High-resolution X-ray spectroscopy with <em>Hitomi</em> was expected to resolve the origin of the faint unidentified <em>E</em> &asymp; 3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the <em>Hitomi</em> first-light observation of the Perseus cluster. The emission line expected for Perseus based on the <em>XMM-Newton</em> signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the <em>Hitomi</em> data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the <em>XMM</em> measurement uncertainties for this region, the inconsistency with <em>Hitomi</em> is at a 99% significance for a broad dark matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-<em>n</em> transitions of S XVI (<em>E</em> ; 3.44 keV rest-frame)&mdash;a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with <em>XMM</em> pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.</p>