  We carried out a systematic search of precursors on the sample of short GRBs
observed by Swift. We found that ~8-10% of short GRBs display such early
episode of emission. One burst (GRB 090510) shows two precursor events, the
former ~13 s and the latter ~0.5 s before the GRB. We did not find any
substantial difference between the precursor and the main GRB emission, and
between short GRBs with and without precursors. We discuss possible mechanisms
to reproduce the observed precursor emission within the scenario of compact
object mergers. The implications of our results on quantum gravity constraints
are also discussed.
