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 $1.0\pm0.3$. The X-ray light curve decays with slope $-2.58\pm0.11$ and shows a small flare in the early phases. The spectrum can be described with a power law with photon index $1.54\pm0.16$ and absorption $(7.5_{-3.2}^{+4.3})\times10^{20}$ 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 $3\times10^{-3}$ cm-3. We also discuss different hypothesis on the possible origin of the flare.