Fermi Gamma-ray Space Telescope

Validation and In-Flight Calibration of the Pass 8 Instrument Response Functions

M. Wood
P. Bruel, on behalf of the LAT Collaboration

Abstract:

Pass 8 represents a multi-year effort to overhaul the Fermi Large Area Telescope (LAT) event reconstruction and classification algorithms and was released to the public in June 2015. A major focus of the Pass 8 effort was improving the accuracy of the instrument simulation and thereby reducing systematic uncertainties on the Instrument Response Functions (IRFs). The improvements to the instrument description achieved with Pass 8 have largely mitigated the need for in-flight corrections to the point spread function (PSF) and the effective area for FRONT and BACK event types. Whereas the three previous public IRF releases have featured in-flight corrections, the current Pass 8 response functions (P8R2_V6) are based entirely on the Monte Carlo description of the instrument. However some residual discrepancies remain in the effective area response functions of the new PSF and EDISP event types which has motivated an effort to develop in-flight corrections for the Pass 8 IRFs. We present validation studies quantifying the systematic uncertainties of the current Pass 8 IRFs and plans for an upcoming Pass 8 IRF release that will include in-flight corrections.