The sterile neutrino ($N_1$) features in multiple extensions of the standard model and is a compelling dark matter candidate, especially as the decay of $N_1$ with mass $m_\mathrm{ s}=7.1\mathrm{ keV}$ is a possible source for the unexplained $3.55\mathrm{ keV}$ 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 $N_1$ and the uncertainty in its parameters. We compare predictions for the $m_{ s}=7.1\mathrm{ keV}$ $N_1$ mixing angle, $\sin^2(2\theta)$, and half-mode mass, $M_\mathrm{ hm}$, as described in the $\nu$MSM standard model extension to existing X-ray observations and structure formation constraints. The strongest available constraints rule out $N_1$ as a dark matter candidate, and a more optimistic reading of the data prefers $\sin^2(2\theta)=5\times10^{ -11}$ and $M_\mathrm{ hm}=3.5\times10^{ 8}M_{ \odot}$. We highlight that the most promising upcoming opportunity for a detection is to find a line of velocity dispersion $\sim500\mathrm{ kms}^{ -1}$ 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 $N_1$.
Categories: astro-ph.CO hep-ph