Secondary radio eclipse of the transiting planet HD 189733 b: an upper limit at 307-347 MHz

We report the first attempt to observe the secondary eclipse of a transiting extra-solar planet at radio wavelengths. We observed HD 189733 b with the Robert C. Byrd Green Bank Telescope of the National Radio Astronomy Observatory (NRAO) over about 5.5 h before, during and after secondary eclipse, at frequencies of 307-347 MHz. In this frequency range, we determine the 3s upper limit to the flux density to be 81 mJy. The data are consistent with no eclipse or a marginal reduction in flux at the time of secondary eclipse in all subsets of our bandwidth; the strongest signal is an apparent eclipse at the 2 sigma level in the 335.2-339.3 MHz region. Our observed upper limit is close to theoretical predictions of the flux density of cyclotron-maser radiation from the planet.