Calibration of AGILE-GRID with in-flight data and Monte Carlo simulations

AGILE is a gamma/X-ray telescope which has been in orbit since 23 April 2007. The gamma-ray detector, AGILE-GRID, has observed Galactic and extragalactic sources, many of which were collected in the first AGILE Catalog. We present the calibration of the AGILE-GRID using in-flight data and updated Monte Carlo simulations, producing response matrices for the effective area, energy dispersion, and point spread dispersion as a function of pointing direction in instrument coordinates and energy. We performed Monte Carlo simulations in GEANT3 at different gamma-ray photon energies and incident angles, using Kalman filter-based photon reconstruction and on-board and on-ground filters. Long integrations of in-flight observations of the Vela, Crab and Geminga sources in broad and narrow energy bands were used to validate and improve the instrument response matrices. The weighted average point spread functions as a function of spectra correspond well to the data for all sources and energy bands. Recent changes in both the implementation of the interpolation of point spread function Monte Carlo data and the procedure for construction of the energy-weighted effective area have improved the correspondence between predicted and observed flux and spectra of celestial calibration sources, reducing false positives and obviating the need for post-hoc energy-dependent scaling factors. These changes will be reflected in the upcoming public release of the AGILE analysis software by the Agile Science Data Centre.