Exoplanet spectroscopy with JWST NIRISS: diagnostics and case studies

The JWST is ushering in a new era in remote sensing of exoplanetary atmospheres. Atmospheric retrievals of exoplanets can be highly sensitive to high-precision JWST data. It is, therefore, imperative to characterize the instruments and noise sources using early observations to enable robust characterization of exoplanetary atmospheres using JWST-quality spectra. This work is a step in that direction, focusing on the Near Infrared Imager and Slitless Spectrograph (NIRISS) Single Object Slitless Spectroscopy (SOSS) 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 of NIRISS 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 of the data reduction and analysis, including sources of contamination, 1/f noise, and system properties such as limb darkening. The slitless nature of NIRISS SOSS makes it susceptible to contamination due to background sources. We present a method to model and correct for dispersed field stars that can significantly improve the accuracy of the observed spectra. In doing so, we also report an empirically determined throughput function for the instrument. We find significant correlated noise in the derived spectra, which may be attributed to 1/f noise, and discuss its implications for spectral binning. We quantify the covariance matrix that would enable the consideration of correlated noise in atmospheric retrievals. Finally, we conduct a comparative assessment of NIRISS SOSS spectra of WASP-39 b reported using different pipelines and highlight important lessons for exoplanet spectroscopy with JWST NIRISS.