Observation of B+ -> p Lambdabar gamma

We report the first observation of the radiative hyperonic B decay B+ -> p Lambdabar gamma, using a 140 fb^{-1} data sample recorded on the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric energy e+e- collider. The measured branching fraction is B(B+ -> p Lambdabar gamma) = (2.16 ^{+0.58}_{-0.53} \pm 0.20) \times 10^{-6}. We examine its M_pLambdabar distribution and observe a peak near threshold. This feature is expected by the short-distance b -> s gamma transition. A search for B+ -> p Sigmabar gamma yields no significant signal and we set a 90% confidence-level upper limit on the branching fraction of B(B+ -> p Sigmabar gamma) < 4.6 \times 10^{-6}.