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Absolute Calibration of the Infrared Array Camera on the Spitzer Space Telescope
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope isabsolutely calibrated by comparing photometry of a set of A stars nearthe north ecliptic pole to predictions based on ground-basedobservations and a stellar atmosphere model. The brightness of pointsources is calibrated to an accuracy of 3%, relative to models forA-star stellar atmospheres, for observations performed and analyzed inthe same manner as for the calibration stars. This includes correctionsfor the location of the star in the array and the location of thecentroid within the peak pixel. Long-term stability of the IRACphotometry was measured by monitoring the brightness of A dwarfs and Kgiants (near the north ecliptic pole) observed several times per month;the photometry is stable to 1.5% (rms) over a year.Intermediate-timescale stability of the IRAC photometry was measured bymonitoring at least one secondary calibrator (near the ecliptic plane)every 12 hr while IRAC was in nominal operations; the intermediate-termphotometry is stable, with a 1% dispersion (rms). One of the secondarycalibrators was found to have significantly time-variable (5%)mid-infrared emission, with a period (7.4 days) matching the opticallight curve; it is possibly a Cepheid variable.
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