GRB 051210: Swift detection of a short gamma ray burst

Aims. The short/hard GRB 051210 was detected and located by the Swift-BAT instrument and rapidly pointed towards by the narrow field instruments. The XRT was able to observe a bright, rapidly fading X-ray emission. We present the analysis of the prompt and afterglow emission of this event. Methods. The BAT spectrum is a power-law with photon index 1.0 +/- 0.3. The X-ray light curve decays with slope -2.58 +/- 0.11 and shows a small flare in the early phases. The spectrum can be described with a power law with photon index 1.54 +/- 0.16 and absorption (7.5(-3.2)(+4.3)) x 10(20) cm(-2). Results. We find that the X-ray emission is consistent with the hypothesis that we are observing the curvature effect of a GRB that occurred in a low density medium, with no detectable afterglow attributable to an external shock. We estimate the density of the circumburst medium to be lower than 3 x 10(-3) cm(-3). We also discuss different hypothesis on the possible origin of the flare.