Search for Very Short Bursts with the AGILE Mini-Calorimeter

In this paper the results of the first months of burst search on very short time scales with the Mini-Calorimeter (MCAL) on-board the AGILE mission are presented. MCAL is equipped with a flexible on-board trigger logic for autonomous GRB detection. In addition to standard trigger time windows, MCAL has been equipped with trigger capabilities on 16 ms, 1 ms and sub-millisecond time windows. Since June 2008 burst search on these short time scales has been enabled, resulting in an average rate of 4 triggers/orbit, compliant with telemetry budget. An on-ground rejection strategy is applied, allowing more sophisticated selection criteria than those allowed by the on-board trigger logic. Triggers due to instrumental effects or geophysical phenomena can be spotted due to their photon-by-photon pattern; what remains after this cleaning is an almost unexplored territory and could shed light on the origin of very short bursts. The trigger criteria, as well as the on-ground rejection strategy and the properties of the selected events will be described and discussed.