Flux Calibration Standards for Webb

Ralph Bohlin, bohlin@stsci.edu, Karl Gordon, kgordon@stsci.edu, and Jason Kalirai, jkalirai@stsci.edu

Flux calibrations in physical units are required to compare observational results with physical models of observed objects. For example, one incentive for accurate absolute flux standards is the need to measure the relative fluxes of red-shifted supernovae (SNe) type Ia spectra in their rest frame. Cosmological parameters derived from SNe Ia observations specify the nature of dark energy and the acceleration of the expansion rate of the universe. The constraints on these parameters are most significant when the relative flux versus wavelength is known to an accuracy of 1% or better. Precise flux calibrations are also needed to accurately measure the Hubble constant using Cepheid variables.

The Institute anticipates that a variety of important science programs of the James Webb Space Telescope will rely on high-quality flux calibrations. For that reason Institute staff are developing a set of stellar spectral energy distributions (SEDs) to facilitate the transfer of the flux calibration to Webb science targets.

The Webb instrument complement comprises Mid-Infrared Instrument (MIRI; 18 modes), Near-Infrared Camera (NIRCAM; 8 modes), Near-Infrared Spectrometer (NIRSPEC; 7 modes), and Tunable Filter Instrument (TFI; 8 modes). These instruments have a wide dynamic range of sensitivity over their combined wavelength coverage of 0.6–30 microns.

Absolute flux calibrations of spectrometers and photometers are normally derived from observations of standard stars with well-known SEDs. Figure 1 shows a sample of SEDs from our Webb standard-star library, along with the range of sensitivities for NIRCam imaging and MIRI imaging and coronagraphy. Each of the standards is established by fitting a model atmosphere to pedigreed Hubble fluxes. The SED beyond the 1-micron limit of Space Telescope Imaging Spectrograph (STIS) or, in some cases, beyond the 2.5-micron limit of Near Infrared Multi-Object Spectrometer (NICMOS), depends on the fidelity of the model atmospheres used to make the best fits at the shorter wavelengths.

Sets of standard stars in two brightness ranges suffice to establish all infrared flux calibrations for the four Webb instruments. A selection of at least three stellar types avoids systematic effects that might occur in modeling only one type. To minimize the complication of molecular species in the models, the three stellar types are early G solar analogs (Bohlin 2010), early A (Bohlin and Cohen 2008), and white dwarf (WD) stars. The A and G stars are the primary standards for Spitzer (Rieke et al. 2008), while the WDs are the primary standards for Hubble (Bohlin et al. 2001). Whenever stars brighter than any known WD are required, OB stars are used instead of WDs. If all three types yield the same Webb instrumental sensitivities, then a precision at the level of agreement can be claimed.