The automatic real-time gamma-ray burst pipeline of the 2 m Liverpool Telescope

The 2 m Liverpool Telescope (LT), owned by Liverpool John Moores University, is located in La Palma (Canary Islands) and operates in fully robotic mode. In 2005, the LT began conducting an automatic gamma-ray burst (GRB) follow-up program. On receiving an automatic GRB alert from a gamma-ray observatory ( Swift, INTEGRAL, HETE-2, or IPN), the LT initiates a special override mode that conducts follow-up observations within 2-3 minutes of the GRB onset. This follow-up procedure begins with an initial sequence of short (10 s) exposures acquired through an band filter. These images are reduced, analyzed, and interpreted automatically using pipeline software developed by our team, called LT-TRAP (Liverpool Telescope Transient Rapid Analysis Pipeline); the automatic detection and successful identification of an unknown and potentially fading optical transient triggers a subsequent multicolor imaging sequence. In the case of a candidate brighter than r' = 15 either a polarimetric (from 2006) or a spectroscopic observation (from 2007) will be triggered on the LT. If no candidate is identified, the telescope continues to obtain z', r', and i' band imaging with increasingly longer exposure times. Here we present a detailed description of the LT-TRAP and briefly discuss the illustrative case of the afterglow of GRB 050502a, whose automatic identification by the LT just 3 minutes after the GRB led to the acquisition of the first early-time (< 1 hr) multicolor light curve of a GRB afterglow.