How Do Spitzer IRAC Fluxes Compare to HST CALSPEC?

An accurate tabulation of stellar brightness in physical units is essential for a multitude of scientific endeavors. The HST/CALSPEC database of flux standards contains many stars with spectral coverage in the 0.115-1 mu m range, with some extensions to longer wavelengths of 1.7 or 2.5 mu m. Modeled flux distributions to 32 mu m for calibration of JWST complement the shorter-wavelength HST measurements. Understanding the differences between IRAC observations and CALSPEC models is important for science that uses IR fluxes from multiple instruments, including JWST. The absolute flux of Spitzer IRAC photometry at 3.6-8 mu m agrees with CALSPEC synthetic photometry to 1% for the three prime HST standards: G191B2B, GD153, and GD71. For a set of 17-22 A-star standards, the average IRAC difference rises from agreement at 3.6 mu m to 3.4% +/- 0.1% brighter than CALSPEC at 8 mu m. For a smaller set of G-type stars, the average of the IRAC photometry falls below CALSPEC by as much as 3.7% +/- 0.3% for IRAC1, while one G-type star, P330E, is consistent with the A-star ensemble of IRAC/CALSPEC ratios.