We use a non-detection in \nu = 1.4 GHz Green Bank Telescope observations of the ultra-faint dwarf spheroidal galaxy Segue I , which could be immersed in a non-negligible halo magnetic field of the Milky Way , to place bounds on particle dark matter properties .
We model the galaxy using an Einasto dark matter profile , and compute the expected synchrotron flux from dark matter annihilation as a function of the magnetic field strength B , diffusion coefficient D _ { 0 } , and particle mass m _ { \chi } for different annihilation channels .
The data strongly disfavor annihilations to e ^ { + } e ^ { - } for m _ { \chi } \lesssim 50 GeV , but are not sensitive to the b \bar { b } channel .
Adopting a fiducial B \sim 2 \mu G inferred from Segue I ’ s proximity to the Milky Way , our models of annihilation to \tau ^ { + } \tau ^ { - } with m _ { \chi } = 30 GeV require an intermediate value of D _ { 0 } for consistency with the data .
The most compelling limits are obtained for WIMP annihilation to \mu ^ { + } \mu ^ { - } : we exclude m _ { \chi } \lesssim 30 GeV \rightarrow \mu ^ { + } \mu ^ { - } at 95 % confidence , unless D _ { 0 } exceeds the Milky Way value or B is significantly smaller than we have assumed .