ExoTiC-ISM: A Python package for marginalised exoplanet transit parameters across a grid of systematic instrument models

Iva Laginja,Hannah Wakeford

doi:10.21105/joss.02281

Abstract

To address the the problem of calibration of instrument systematics in transit light curves, we present the Python package ExoTiC-ISM. Transit spectroscopy can reveal many different chemical components in exoplanet atmospheres, but such results depend on well-calibrated transit light curve observations. Each transit data set will contain instrument systematics that depend on the instrument used and will need to be calibrated out with an instrument systematic model. The proposed solution in Wakeford et al. (2016) (arXiv:1601.02587 [astro-ph.EP]) is to use a marginalisation across a grid of systematic models in order to retrieve marginalised transit parameters. Doing this over observations in multiple wavelengths yields a robust transmission spectrum of an exoplanet. ExoTiC-ISM provides tools to perform this analysis, and its current capability contains a systematic grid that is applicable to the Wide Field Camera 3 (WFC3) detector on the Hubble Space Telescope (HST), particularly for the two infrared grisms G141 and G102. By modularisation of the code and implementation of more systematic grids, ExoTiC-ISM can be used for other instruments, and an implementation for select detectors on the James Webb Space Telescope (JWST) will provide robust transit spectra in the future.

Highlights

There are many different chemical components that transit spectroscopy can reveal in an exoplanet, but a majority of the exoplanets studied via transmission spectroscopy are on close-in orbits around their stars lasting only several days, most of them giant Jupiter- or Neptune-sized worlds
The solution that was applied to WFC3 data in Wakeford, Sing, Evans, Deming, & Mandell (2016) performs a marginalisation across a grid of systematic models that take different corrections across an exoplanet transit data set into account
Following the method proposed by Gibson (2014), a Levenberg-Marquardt least-squares minimisation is performed across all systematic models, which yields a set of fitted transit parameters for each systematic model

Summary

Transit spectroscopy of exoplanets

There have been a slew of planet detections outside our own solar system over the past two decades and several characterisation methods can be used to determine their chemical compositions. With this technique, astronomers measure the star light passing through an exoplanet’s atmosphere while it is transiting in front of its host star. There are many different chemical components that transit spectroscopy can reveal in an exoplanet, but a majority of the exoplanets studied via transmission spectroscopy are on close-in orbits around their stars lasting only several days, most of them giant Jupiter- or Neptune-sized worlds. For these giant, close-in exoplanets, the most dominant source of absorption will be from water vapour, which is expected to be well-mixed throughout their atmosphere. To measure H2O in the atmospheres of exoplanets, astronomers use the Hubble Space Telescope’s Wide Field Camera 3 (HST WFC3) infrared capabilities to detect the absorption signatures of H2O at 0.9 μm with the G102 grism, and at 1.4 μm with the G141 grism (e.g. Kreidberg et al, 2015; Sing et al, 2016; Spake et al, 2018; Wakeford et al, 2018, 2017)

Calibration of instrument systematics with marginalisation

Dependencies and usage