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Exoplanet Spectroscopy with JWST NIRISS: Diagnostics and Case Studies Published: 6/7/2023 9:00:00 PM Updated: 6/7/2023 9:00:00 PM

Paper abstract: The James Webb Space Telescope (JWST) is ushering in a new era in remotesensing of exoplanetary atmospheres. Atmospheric retrievals of exoplanets canbe highly sensitive to high-precision JWST data. It is, therefore, imperativeto characterise the instruments and noise sources using early observations toenable robust characterisation of exoplanetary atmospheres using JWST-qualityspectra. The present work is a step in that direction, focusing on the NIRISSSOSS instrument mode, with a wavelength coverage of 0.6 - 2.8 {\mu}m and R ~700. Using a custom-built pipeline, JExoRes, we investigate key diagnostics ofNIRISS SOSS with observations of two giant exoplanets, WASP-39 b and WASP-96 b,as case studies. We conduct a detailed evaluation of the different aspects ofthe data reduction and analysis, including sources of contamination, 1/f noise,and system properties such as limb darkening. The slitless nature of NIRISSSOSS makes it susceptible to contamination due to background sources. Wepresent a method to model and correct for dispersed field stars which cansignificantly improve the accuracy of the observed spectra. In doing so, wealso report an empirically determined throughput function for the instrument.We find significant correlated noise in the derived spectra, which may beattributed to 1/f noise, and discuss its implications for spectral binning. Wequantify the covariance matrix which would enable the consideration ofcorrelated noise in atmospheric retrievals. Finally, we conduct a comparativeassessment of NIRISS SOSS spectra of WASP-39 b reported using differentpipelines and highlight important lessons for exoplanet spectroscopy with JWSTNIRISS.