JWST Noise Floor II: Systematic Error Sources in JWST NIRCam Time Series

TL;DR

This paper analyzes systematic error sources in JWST NIRCam time series observations, estimating a low noise floor of about 9 ppm, crucial for exoplanet atmosphere characterization, while highlighting potential unknown errors and calibration challenges.

Contribution

It provides a detailed assessment of known systematic errors in JWST NIRCam time series, estimating their combined impact and emphasizing the importance of correction strategies for high-precision exoplanet studies.

Findings

Pointing jitter and antenna moves cause less than 6 ppm variation.

Thermal instabilities contribute less than 2 ppm to noise.

Total estimated systematic noise floor is approximately 9 ppm per visit.

Abstract

JWST holds great promise in characterizing atmospheres of transiting exoplanets, potentially providing insights into Earth-sized planets within the habitable zones of M dwarf host stars if photon-limited performance can be achieved. Here, we discuss the systematic error sources that are expected to be present in grism time series observations with the NIRCam instrument. We find that pointing jitter and high gain antenna moves on top of the detectors' subpixel crosshatch patterns will produce relatively small variations (less than 6 parts per million, ppm). The time-dependent aperture losses due to thermal instabilities in the optics can also be kept to below 2 ppm. To achieve these low noise sources, it is important to employ a sufficiently large (more than 1.1 arcseconds) extraction aperture. Persistence due to charge trapping will have a minor (less than 3 ppm) effect on time series 20 minutes into an exposure and is expected to play a much smaller role than it does for the HST WFC3 detectors. We expect temperature fluctuations to be less than 3 ppm. In total, our estimated noise floor from known systematic error sources is only 9 ppm per visit. We do however urge caution as unknown systematic error sources could be present in flight and will only be measurable on astrophysical sources like quiescent stars. We find that reciprocity failure may introduce a perennial instrument offset at the 40 ppm level, so corrections may be needed when stitching together a multi-instrument multi-observatory spectrum over wide wavelength ranges.