Sterile neutrinos in the Milky Way: Observational constraints

TL;DR

This study tests sterile neutrino dark matter via the X-ray decay line hypothesis by analyzing Chandra blank-sky data that probe the Milky Way halo. By modeling the halo with an NFW profile and deriving conservative upper limits on a monochromatic decay line across the 0.3–9 keV band, the authors translate flux limits into constraints on the decay rate and, hence, on the neutrino mass $m_s$, mixing angle $\sin^2(2\theta)$, and entropy production $S$. They find that while substantial entropy production is disfavored in certain mass ranges, a broad region of parameter space remains viable, keeping sterile neutrinos as a plausible dark matter candidate. The results complement Ly$\alpha$ and other cosmological constraints, and the authors note that more stringent or model-dependent limits could be obtained with refined background modeling and independent X-ray observations.

Abstract

We consider the possibility of constraining decaying dark matter by looking out through the Milky Way halo. Specifically we use Chandra blank sky observations to constrain the parameter space of sterile neutrinos. We find that a broad band in parameter space is still open, leaving the sterile neutrino as an excellent dark matter candidate.

Figures (4)

• Figure 1: The blank sky ACIS-S3 spectrum -- i.e. a view through the Milky Way halo. Also shown is the maximal single gaussian emission line from decaying dark matter at a specific energy as discussed in Section \ref{['sec:analysis']}.
• Figure 2: The decay rate as function of photon energy. The solid line is the upper limit from the Chandra blank sky observations viewing out through the Milky Way halo.
• Figure 3: The lifetime constrained from the flux of the Chandra blank sky data (red) and A383 (green). The $\nu$MSM prediction for $S=1$ and $g_*=10-20$ (hatched) and several variations of $S$ and $g_*$ (black) have been overplotted.
• Figure 4: The $m_s-S$ parameter space with constraints from the Chandra blank sky data (red, fat, diagonal, this paper), Ly-$\alpha$ observations (blue, diagonal, conservative viel, and green, vertical, ambitious seljak), Tremaine-Gunn bound $m>0.5$ keV (black, horizontal).