Can neutron stars constrain dark matter?

Abstract

Because of their strong gravitational field, neutron stars capture weakly interacting dark matter particles (WIMPs) more efficiently compared to other stars, including the white dwarfs. Once captured, the WIMPs sink to the neutron star center and annihilate, heating the star. We find that this heat could lead to detectable effects on the surface temperature of old neutron stars, especially those in dark-matter-rich regions such as the Galactic center or cores of globular clusters. The capture and annihilation is fully efficient even for WIMP-to-nucleon cross sections (elastic or inelastic) as low as $\ensuremath{\sim}{10}^{\ensuremath{-}45}\text{ }\text{ }{\mathrm{cm}}^{2}$, and for the annihilation cross sections as small as $\ensuremath{\sim}{10}^{\ensuremath{-}57}\text{ }\text{ }{\mathrm{cm}}^{2}$. Thus, detection of a sufficiently cold neutron star in a dark-matter-rich environment would exclude a wide range of dark matter candidates, including those with extremely small cross sections.