Constraints on dark matter annihilation from the FAST observation of the Coma Berenices dwarf galaxy

The Galactic center 7-ray excess, detected by the Fermi-LAT, is a very attractive tentative signal from dark matter annihilation. Searching for associated synchrotron emissions can test the dark matter interpretation for this excess. We geared the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) toward Coma Berenices (a dwarf spheroidal galaxy) for two hours of observation, and found no significant continuum radio emission, which could put constraints on the dark matter annihilation cross section, from our target. We set very stringent annihilation cross section constraints, with roughly an order of magnitude improvement over a large range of masses compared with previous radio searches. The dark matter scenario for the Galactic center 7-ray excess is in tension with the FAST observation for reasonable choices of astrophysical factors. But considering the large uncertainty on astrophysical parameters, such as the magnetic field, the diffusion coefficient, and the diffusion radius, and on dark matter halo parameters, the dark matter interpretation for the excess could still survive. Further radio observations by the FAST and other radio telescopes may reveal more about the dark matter properties.