Marginalizing instrument systematics in HST WFC3 transit light curves
Sing, David K.
American Astronomical Society
This is the final version of the article. Available from American Astronomical Society via the DOI in this record.
Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) infrared observations at 1.1–1.7 μm probe primarily the H2O absorption band at 1.4 μm, and have provided low-resolution transmission spectra for a wide range of exoplanets. We present the application of marginalization based on Gibson to analyze exoplanet transit light curves obtained from HST WFC3 to better determine important transit parameters such as Rp/R*, which are important for accurate detections of H2O. We approximate the evidence, often referred to as the marginal likelihood, for a grid of systematic models using the Akaike Information Criterion. We then calculate the evidence-based weight assigned to each systematic model and use the information from all tested models to calculate the final marginalized transit parameters for both the band-integrated and spectroscopic light curves to construct the transmission spectrum. We find that a majority of the highest weight models contain a correction for a linear trend in time as well as corrections related to HST orbital phase. We additionally test the dependence on the shift in spectral wavelength position over the course of the observations and find that spectroscopic wavelength shifts dl (l) best describe the associated systematic in the spectroscopic light curves for most targets while fast scan rate observations of bright targets require an additional level of processing to produce a robust transmission spectrum. The use of marginalization allows for transparent interpretation and understanding of the instrument and the impact of each systematic evaluated statistically for each data set, expanding the ability to make true and comprehensive comparisons between exoplanet atmospheres.
The authors would like to thank N. Gibson for useful comments and discussions on this paper and the analysis technique presented. H.R. Wakeford acknowledges support by an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. H.R. Wakeford, D.K. Sing, and T. Evans acknowledge funding from the European Research Council under the European Unions Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 336792. This work is based on observations made with the NASA/ESA Hubble Space Telescope. This research has made use of NASAs Astrophysics Data System and components of the IDL astronomy library.
Vol. 819, No. 1