The effect of spectroscopic binning on atmospheric retrievals

TL;DR

This study investigates how spectroscopic binning affects atmospheric retrieval accuracy for JWST data, highlighting the importance of resolution in resolving atmospheric features and degeneracies, especially in cloudy conditions.

Contribution

It provides a systematic analysis of the impact of spectral resolution and binning on retrieval accuracy using simulated JWST observations of WASP-39b, including cloud effects.

Findings

Higher resolution is necessary for atmospheres with high cloud decks.

Large safe zones exist where accurate retrievals are possible.

Resolution needed to resolve parameter degeneracies exceeds that for individual parameter constraints.

Abstract

With the JWST offering higher resolution data in space-based transmission spectroscopy, understanding the capabilities of our current atmospheric retrieval pipelines is essential. These new data cover wider wavelength ranges and at much higher spectral resolution than previous instruments have been able to offer. Therefore, it is often appealing to bin spectra to fewer points, better constrained in their transit depth, before using them as inputs for atmospheric retrievals. As such, we produce a simulation replicating the observations of WASP-39b by the Near Infrared Spectrograph (NIRSpec) instrument on board JWST using the PRISM dispersion element. Then, we assess the accuracy and consistency of retrievals while varying both the resolution and the average photometric error of this simulated spectrum. We repeat this analysis on three different simulation setups where each includes an opaque cloud layer at a different height in the atmosphere. In agreement with previous studies, we find that a much greater resolution is needed in the case of a high cloud deck since features are already heavily muted by the presence of the clouds. In the other two cases, there are large 'safe zones' in the parameter space where accurate estimations are made. If these maps can be generalized, they could be used to inform future observations on how long to observe a given target in order to achieve the most accurate retrieval results. We also find that the resolution required to fully resolve the degeneracies between the parameters contributing to the spectra is much greater than that needed to constrain the marginalized posterior distributions for each parameter individually.