Awesome SOSS: Transmission Spectroscopy of WASP-96b with NIRISS/SOSS

TL;DR

This paper demonstrates the use of JWST's SOSS mode for transmission spectroscopy of exoplanet WASP-96b, detailing data reduction techniques and revealing atmospheric properties such as metallicity and scattering features.

Contribution

It provides the first detailed data reduction guide for JWST's SOSS mode and presents transmission spectra analysis of WASP-96b, highlighting its atmospheric composition.

Findings

Atmosphere metallicity between 1x and 5x solar

No grey cloud deck needed to fit the spectrum

Evidence of scattering or Na feature shortward of 0.9μm

Abstract

The future is now - after its long-awaited launch in December 2021, JWST began science operations in July 2022 and is already revolutionizing exoplanet astronomy. The Early Release Observations (ERO) program was designed to provide the first images and spectra from JWST, covering a multitude of science cases and using multiple modes of each on-board instrument. Here, we present transmission spectroscopy observations of the hot-Saturn WASP-96b with the Single Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph, observed as part of the ERO program. As the SOSS mode presents some unique data reduction challenges, we provide an in-depth walk-through of the major steps necessary for the reduction of SOSS data: including background subtraction, correction of 1/f noise, and treatment of the trace order overlap. We furthermore offer potential routes to correct for field star contamination, which can occur due to the SOSS mode's slitless nature. By comparing our extracted transmission spectrum with grids of atmosphere models, we find an atmosphere metallicity between 1x and 5x solar, and a solar carbon-to-oxygen ratio. Moreover, our models indicate that no grey cloud deck is required to fit WASP-96b's transmission spectrum, but find evidence for a slope shortward of 0.9$\mu$m, which could either be caused by enhanced Rayleigh scattering or the red wing of a pressure-broadened Na feature. Our work demonstrates the unique capabilities of the SOSS mode for exoplanet transmission spectroscopy and presents a step-by-step reduction guide for this new and exciting instrument.