Analyzing Exoplanet Transits Observed with the WFC3/UVIS G280 Grism

TL;DR

The paper addresses reducing exoplanet transit data obtained with the WFC3/UVIS G280 grism, a UV-optical channel that enables atmospheric studies but presents challenges such as curved traces and overlapping orders. It presents a Jupyter notebook and a Python toolkit that guide users from downloading calibrated flt files through background subtraction, cosmic-ray correction, embedding, spectral trace fitting, spectral extraction, and generation of broadband and spectroscopic transit light curves. The outputs provide ready-to-fit light curves for deriving planetary transmission spectra, compatible with public fitting tools like PacMan, WFC3, and Eureka. The work lowers barriers for applying G280 data to atmospheric characterization, offering practical procedures and reproducible workflows for cloud/haze, UV absorbers, and photochemistry analyses.

Abstract

Here we describe a Jupyter notebook demonstrating methods for the reduction and analysis of exoplanet transit observations taken with the WFC3/UVIS G280 grism. Released on Space Telescope's hst_notebooks GitHub repository, this notebook presents an example workflow for processing time-series observations taken with the G280 grism - from the calibrated flat-fielded spectra to transit light curves ready for fitting. The specific routines presented in the notebook are explained here, and are meant to highlight data reduction steps that users will typically apply to extract transit light curves. The steps include background subtraction, spatial and temporal cosmic ray correction, spectral trace fitting, spectral extraction, and light curve generation. The end products of the routines in the Jupyter notebook are the raw broadband and spectroscopic light curves, which can be ingested into publicly available light curve fitting tools to extract planetary transmission spectra.

Figures (8)

• Figure 1: The full-frame G280 sky image for UVIS2 with the subarray size and location outlined in blue.
• Figure 2: The extracted white light curve for HAT-P-41b using a histogram background subtraction method (left) compared to a G280 sky background subtraction (right).
• Figure 3: The calibrated 2D corrected image (top) compared to the background-subtracted and cosmic-ray-corrected image (bottom).
• Figure 4: The cleaned, cosmic-ray-corrected subarray image (top) compared to the cleaned and embedded full-frame image (bottom).
• Figure 5: The fitted spectral trace (red) for the $+1$ (top) and $-1$ (bottom) G280 orders.