How do Spitzer IRAC Fluxes Compare to HST CALSPEC

TL;DR

This study compares Spitzer IRAC flux measurements with HST CALSPEC models across multiple stars, finding high agreement at shorter wavelengths and small systematic differences at longer wavelengths, which is crucial for multi-instrument astrophysical research.

Contribution

It provides a detailed comparison between IRAC fluxes and CALSPEC models, highlighting wavelength-dependent agreement and discrepancies for different stellar standards.

Findings

IRAC fluxes agree with CALSPEC to within 1% at 3.6 microns.

Discrepancies increase to about 3.4% at 8 microns for A-star standards.

G-star fluxes show a slight offset, with some falling below CALSPEC by up to 3.7%.

Abstract

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 \micron\ range with some extensions to longer wavelengths of 1.7 or 2.5 \micron. Modeled flux distributions to 32 \micron\ 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 \micron\ 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 \micron\ to 3.4 $\pm$0.1\% brighter than CALSPEC at 8 \micron. For a smaller set of G-stars, the average of the IRAC photometry falls below CALSPEC by as much as 3.7 $\pm$0.3\% for IRAC1, while one G-star, P330E, is consistent with the A-star ensemble of IRAC/CALSPEC ratios.