The topological trigger system for the VERITAS upgrade

VERITAS (Very Energetic Radiation Imaging Telescope Array System) is an array of four IACTs (Imaging Air Cherenkov Telescopes) located south of Tucson, Arizona, USA. VERITAS observes the Northern Hemisphere sky in VHE (E>100 GeV) gamma rays. A major upgrade of the VERITAS trigger electronics and cameras is currently underway. The upgrade aims to increase the sensitivity and lower the energy threshold of the instrument. The energy threshold can potentially be lowered by more stringent timing discrimination of accidental triggers due to night sky background (NSB). The new VERITAS FPGA-based pattern trigger has been in operation since Novemeber 2011 and provides more stringent timing discrimination by narrowing the required coincidence gate to as low as 3ns. Technical details and operational status of the new VERITAS pattern trigger will be detailed in this work.