One of the key predictions of the WIMP paradigm for Dark Matter (DM) is that
DM particles can annihilate into charged particles. These annihilations will
proceed in e.g. Galactic subhalos such as dwarf Galaxies or, as recently
pointed out, high velocity clouds such as the "Smith Cloud". In this note, we
focus on the radio emission associated with DM annihilations into electrons and
positrons occurring in the Smith Cloud. The phenomenology of this emission is
discussed in quite some detail. We argue that the uncertainties in the
propagation can be captured by the typical diffusion-loss length parameter
(Syrovatskii variable) but that the angle-integrated radio fluxes are
independent of the propagation. We conclude that if the Smith Cloud is indeed
dominated by DM, radio signals from DM annihilation stand out amongst other
messengers. Furthermore, low frequencies such as the ones observed by e.g. the
Low Frequency Array (LOFAR) and the next-generation Square Kilometre Array
(SKA) are optimal for searches for DM in the Smith Cloud. As a practical
application, we set conservative constraints on dark matter annihilation cross
section using data of continuum radio emission from the Galaxy at 22 MHz and at
1.4 GHz. Stronger constraints could be reached by background subtraction,
exploiting the profile and frequency dependence of the putative DM signal. We
set stronger but tentative limits using the median noise in brightness
temperature from the Green Bank Telescope and the LOFAR sensitivities.Comment: 23 pages, 17 figures. Extended discussion on how the limits/forecasts
are obtained. Matches published versio
