Observation of B+ --plambdagamma

We report the first observation of the radiative hyperonic B decay B+ --plambdagamma, using a 140 fb(-1) data sample recorded on upsilom(4S) resonance with the Belle detector at the KEKB asymmetric energy e+e- collider. The measured branching fraction is [symbol: see text](B+ --plambdagamma) = (2.16(+0.58)(-0.53) +/- 0.20) x 10(-6). We examine its M(plambda) distribution and observe a peak near threshold. This feature is expected by the short-distance b --sgamma transition. A search for B+ --pepsilon0gamma yields no significant signal, and we set a 90% confidence-level upper limit on the branching fraction of [symbol: see textB+ --pepsilon0gamma4.6 x 10(-6).