2023MNRAS.524.6345L

The sterile neutrino (N₁) features in multiple extensions of the standard model and is a compelling dark matter candidate, especially as the decay of N₁ with mass m_s = 7.1 keV is a possible source for the unexplained 3.55 keV X-ray line reported in galaxy clusters. This particle will be accessible to the XRISM X-ray mission over the next 12 months. We revisit the physics behind N₁ and the uncertainty in its parameters. We compare predictions for the m_s = 7.1 keV N₁ mixing angle, $\sin ^{2}(2\theta)\,$, and half-mode mass, M_hm, as described in the neutrino minimal standard model (νMSM) extension to existing X-ray observations and structure formation constraints. The strongest available constraints rule out N₁ as a dark matter candidate, and a more optimistic reading of the data prefers $\sin ^{2}(2\theta)\,$ = 5 × 10^–11 and M_hm = 3.5 × 10⁸${\, \rm M_\odot }$. We highlight that the most promising upcoming opportunity for a detection is to find a line of velocity dispersion ∼500 $\, {\rm km}\, {\rm s}^{-1}$ in the Virgo cluster with XRISM, and then draw up a list of future objects of study to determine: (i) whether the line is from dark matter generally, and (ii) if from dark matter, whether that candidate is indeed N₁.