2008 IceCube Update - Section 7
VII. DATA TRIGGERING AND FILTERING
Data collected by the DOMs and sent to the surface is time-sorted, combined into a single stream, and then monitored by a software trigger. IceCube uses two trigger criteria and may add a third. The main trigger is based on multiplicity; it selects time intervals where eight DOMs (with local coincidences) fired within 5 μs. This collects most of the neutrino events. In 2008, a string trigger was added; it selects time intervals when five out of seven adjacent DOMs fired within 1.5 μs. TThis trigger has improved sensitivity for low energy (down to 100 GeV) events, especially upward going muons. A third, 'topological' trigger is also under consideration; it will be optimized for low-energy horizontal muons. When any trigger occurs, all data within the ±10 μs trigger window is saved, becoming an event. If multiple trigger windows overlap, then all of the data from the ΟRed time intervals are saved as a single event.
The total trigger rate (for 40 strings) is about 1,400 Hz. The majority of the triggers (about 1,000 Hz) are due to cosmic-ray muons, with the rest divided among other sources, including IceTop.
Triggered data is reconstructed by an on-line filter system and selected events are transmitted via satellite to the Northern hemisphere. The filters use simple criteria, 'first-guess' reconstruction algorithms and simplified maximum likelihood fitting. Current filters select upward going muons, cascades (νe, ντ and all-flavor neutral current interactions), extremely high energy events, starting and stopping events, and air showers seen in IceTop. Currently, about 6% of the events are selected by these filters, comprising about 32.5 Gbytes/day. The remainder of the data is stored on tapes at the South Pole station. The tapes are carried north during the austral summer.