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The Infrared Array Camera (IRAC) on the Spitzer Space Telescope is absolutelycalibrated by comparing photometry on a set of A stars near the north eclipticpole to predictions based on ground-based observations and a stellar atmospheremodel. The brightness of point sources is calibrated to an accuracy of 3%,relative to models for A star stellar atmospheres, for observations performedand analyzed in the same manner as the calibration stars. This includescorrections for location of the star in the array and the location if thecentroid within the peak pixel. Long-term stability of the IRAC photometry wasmeasured by monitoring the brightness of A dwarfs and K giants (near the northecliptic pole) observed several times per month; the photometry is stable to1.5% (rms) over a year. Intermediate-time-scale stability of the IRACphotometry was measured by monitoring at least one secondary calibrator (nearthe ecliptic plane) every 12 hr while IRAC is in nominal operations; theintermediate-term photometry is stable with a 1% dispersion (rms). One of thesecondary calibrators was found to have significantly time-variable (5%)mid-infrared emission, with period (7.4 days) matching the optical light curve;it is possibly a Cepheid variable.Comment: accepted to PASP; 7/7/05: fixed mistake on page 17 (extended source  corrections must be multiplied not divided

Absolute Calibration of the Infrared Array Camera on theSpitzer Space Telescope