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Arxiv: Exoplanet Transit Spectroscopy with JWST NIRSpec: Diagnostics and Homogeneous Case Study of WASP-39 b Published: 5/10/2024 9:00:00 PM Updated: 5/10/2024 9:00:00 PM

Paper abstract: The JWST has ushered in a new era of exoplanet transit spectroscopy. Amongthe JWST instruments, the Near-Infrared Spectrograph (NIRSpec) has the mostextensive set of configurations for exoplanet time series observations. TheNIRSpec Prism and G395H grating represent two extremes in NIRSpec instrumentmodes, with the Prism spanning a wider spectral range (0.6-5.3 \mum at lowerresolution (R~100) compared to G395H (2.87-5.14 \mum; R~2700). Inthis work, we develop a new data reduction framework, JexoPipe, to conduct ahomogeneous assessment of the two NIRSpec modes for exoplanet spectroscopy. Weuse observations of the hot Saturn WASP-39 b obtained as part of the JWSTTransiting Exoplanets ERS program to assess the spectral quality and stabilitybetween the two instrument modes at different epochs. We explore the noisesources, effect of saturation, and offsets in transmission spectra between thedifferent instrument modes and also between the two G395H NRS detectors. Wefind an inter-detector offset in G395H of ~ 40-50 ppm, consistent withrecent studies. We find evidence for correlated noise in the Prism white lightcurve. We find the G395H spectrum to be of higher precision compared to thePrism at the same resolution. We also compare the JexoPipe spectra with thosereported from other pipelines. Our work underscores the need for robustassessment of instrument performance and identification of optimal practicesfor JWST data reduction and analyses.