Celestial-body focused dark matter annihilation throughout the Galaxy

Indirect detection experiments typically measure the flux of annihilating dark matter (DM) particles propagating freely through galactic halos. We consider a new scenario where celestial bodies “focus” DM annihilation events, increasing the efficiency of halo annihilation. In this setup, DM is first captured by celestial bodies, such as neutron stars or brown dwarfs, and then annihilates within them. If DM annihilates to sufficiently long-lived particles, they can escape and subsequently decay into detectable radiation. This produces a distinctive annihilation morphology, which scales as the product of the DM and celestial body densities, rather than as DM density squared. We show that this signal can dominate over the halo annihilation rate in $\gamma$-ray observations in both the Milky Way Galactic center and globular clusters. We use Fermi and H.E.S.S. data to constrain the DM-nucleon scattering cross section, setting powerful new limits down to $\sim {10}^{-39}{\mathrm{cm}}^2$ for sub-GeV DM using brown dwarfs, which is up to 9 orders of magnitude stronger than existing limits. We demonstrate that neutron stars can set limits for TeV-scale DM down to about ${10}^{-47}{\mathrm{cm}}^2$.

Figure 1

Comparison of the approximate cross sections producing efficient capture (at 99% of maximum capture rate) at the local position for the Sun, brown dwarfs, and neutron stars, as a function of DM mass. Cross sections above these values produce comparable DM annihilation rates. We show a benchmark brown dwarf with radius that of Jupiter, and mass $0.0378M_{\odot }$. The neutron star benchmark has a radius of 10 km and mass $1.4M_{\odot }$.

Figure 2

Maximum DM mass capture rates for a single neutron star or brown dwarf as a function of radius ($r$) from the Galactic center. Results are shown for NSs with $R_{\mathrm{NS}}=10\mathrm{km}$ and $M_{\mathrm{NS}}=1.4M_{\odot }$, and BDs with $M_{\mathrm{BD}}=0.0378M_{\odot }$ and $R_{\mathrm{BD}}=R_J$ (where $R_J$ is the radius of Jupiter). We show varied results for generalized NFW DM profiles, with $\gamma =1.0$, 1.5.

Figure 3

NS-focused annihilation and BD-focused annihilation (solid) vs halo annihilation in the Milky Way Galactic center, for $s$-wave and $p$-wave DM rates (dashed) for varying DM mass. This plot assumes the maximum capture rates for BD/NS. Two panels for different NFW slope $\gamma =1.0$ and 1.5 are shown.

Figure 4

Maximum $E^{2}d\mathrm{\Phi }/dE$ values (at $E_{\gamma }\approx m_{\chi }$) for the Galactic center population of brown dwarfs or neutron stars, for DM densities described by NFW profiles with indices of 1.0 or 1.5, as labeled. The Galactic center gamma-ray fluxes measured by Fermi and H.E.S.S. are shown for comparison, where $E_{\gamma }\approx m_{\chi }$ is assumed (see text for details).

Figure 5

Scattering cross section limits for DM annihilation to long-lived mediators decaying to $\gamma \gamma$ in brown dwarfs (using Fermi), the Sun (using Fermi and HAWC [45, 47]), and neutron stars (using H.E.S.S.). The BD and NS limits are new calculations in this work, calculated using the full Galactic center population of BDs or NSs. The $\gamma =1.0$, 1.5 values correspond to the inner slope of the DM density profile. Our limits have some assumptions; see text for details.

Figure 6

NS-focused annihilation vs halo annihilation signals in the globular cluster Tuc 47, for $s$-wave and $p$-wave DM, for varied DM masses.