Strong Upper Limits on Sterile Neutrino Warm Dark Matter

Sterile neutrinos are attractive dark matter candidates. Their parameter space of mass and mixing angle has not yet been fully tested despite intensive efforts that exploit their gravitational clustering properties and radiative decays. We use the limits on gamma-ray line emission from the Galactic center region obtained with the SPI spectrometer on the INTEGRAL satellite to set new constraints, which improve on the earlier bounds on mixing by more than 2 orders of magnitude, and thus strongly restrict a wide and interesting range of models.

Figure 1

The sterile neutrino dark matter mass $m_s$ and mixing ${sin}^{2}2\theta$ parameter space, with shaded regions excluded. The strongest radiative decay bounds are shown, labeled as Milky Way (this Letter), CXB [11], and x-ray Limits (summarized using Ref. 12; the others [13] are comparable). The strongest cosmological bounds [9] are shown by the horizontal band (see caveats in the text). The excluded Dodelson-Widrow [3] model is shown by the solid line; rightward, the dark matter density is too high (stripes). The dotted lines are models from Ref. 14, now truncated by our constraints.

Figure 2

Left: The line of sight integral $\mathcal{J}(\psi )$ as a function of the pointing angle $\psi$ with respect to the Galactic center direction for the three different profiles considered (Kravtsov, NFW, and Moore, in order of solid line, dashed line, and dotted line). Right: Integrals up to the angle $\psi$ of $\mathcal{J}(\psi )$ (thin upper lines) and $\mathcal{J}(\psi )-\mathcal{J}(30°)$ (thick lower lines). The gray line at 13° marks the field of view for the INTEGRAL flux limit, and we chose ${\int }_{\Delta \Omega }d\Omega [\mathcal{J}(\psi )-\mathcal{J}(30°)]\simeq 0.5$ as a conservative value for our subsequent constraints.