GRB090510: a short bright and hard GRB detected by Fermi

On 2009 May 10, 00:22:59 UT $(T0)$ the Fermi Gamma‐ray Burst Monitor (GBM) triggered and located the short and very bright GRB090510. For the first time, this hard GRB, with an Epeak of few MeV, also triggered independently the Fermi Large Area Telescope (LAT). Swift detected this GRB and the accurate position provided by the Swift/UVOT made possible a spectroscopic redshift measurement of $z = 0.903$ with VLT/FORS2. This short GRB exhibits new features for this kind of events such an extra component (power‐law) at high energies and a long lasting (few minutes) emission observed by the LAT. These observations allow the derivation of very important physical parameters such as the minimum value of the bulk Lorentz factor and they put some unprecedent limits on the dependence of the speed of photons on their energy.