Upgrades to the Event Simulation and Reconstruction for the Fermi Large Area Telescope

The pre-launch event simulation and reconstruction performed beyond our expectations for real data, essentially without modification, and made possible the immediate start of science analysis. But the on-orbit data exhibit unanticipated features that necessitate upgrades to both the simulation (essentially complete) and the reconstruction (ongoing). The major new effect encountered on orbit is the presence of "ghosts," that is, remnant detector response to particles passing through the detector before the particle that triggered the event. These ghosts appear primarily in the form of extra tracks and/or energy deposits. As part of this upgrade, we plan to enhance our ability to discriminate against background particles by introducing additional analysis during the reconstruction phase. We present a description of the effect of ghosts, and of the work needed to deal with them, done and planned, as well as some other ideas for improving the reconstruction.