Mitigating Resampling Artifacts for the JWST IFU Spectrometers with Adaptive Trace Modeling

Abstract

The integral-field unit (IFU) spectrometers on board the James Webb Space Telescope (JWST) undersample the nearly diffraction-limited point spread function provided by the telescope optics. This undersampling produces large oscillating spectral artifacts when the data is resampled into regularly-gridded data cubes, which poses a significant challenge for many scientific analyses. We describe here a generalized technique to use cubic basis spline models to interpolate the observed spectral traces onto a higher-resolution grid prior to data cube rectification, which largely eliminates these artifacts in addition to helping reduce biases in point source spectra from clusters of bad pixels. We demonstrate the utility of this adaptive resampling technique for a variety of JWST NIRSpec and MIRI MRS observations ranging from isolated point sources to embedded AGN, crowded stellar fields with diffuse emission, and protostars with rich molecular bands.

Figures (15)

• Figure 1: Left panel: NIRSpec NRS1 calibrated detector image of spectrophotometric standard star J1757132 (spectral type A8Vm) in the G395H/F290LP spectral configuration. Image uses a logarithmic stretch to show the dispersed trace of the star illuminating multiple of the 30 separate IFU slices. Right panel: Zoom-in on regions of the PSF core and wings illustrating the change in pixel-phase sampling of the PSF as a function of wavelength. Zoomed panels use a variable linear stretch to accentuate details. The across-slice coordinate $\beta$ changes between slices, while the along-slice coordinate $\alpha$ changes within each slice.
• Figure 2: Flowchart outlining the adaptive trace model method for oversampling a given exposure. Each IFU slice is treated independently. Spline models are created only for slices with significant emission, and used only for along-slice locations with steep gradients indicative of undersampling.
• Figure 3: Panel A: Simulated JWST 2$\micron$ PSF illustrating the presence of Airy rings and other complex features due to the telescope optics, with illustrative slice boundaries shown in white. Panels B-F: Real detector profiles as a function of the along-slice coordinate $\alpha$ for observations of standard star J1743045 in various NIRSpec grating settings, slices, and wavelengths. Solid green squares represent the calibrated detector data in the indicated column, solid black points represent data from the neighboring $\pm$50 columns normalized to a common summed value. The orange line represents the derived basis-spline fit to the black points.
• Figure 4: Top panel: Zoom-in of a NIRSpec oversampled detector image showing missing full-field line emission (narrow vertical features) in the vicinity of the lower stellar spectral trace (horizontal features) that has been oversampled using the basis-spline technique. In contrast, the upper traces oversampled using the linear technique show no such missing emission. Bottom panel: Same oversampled image after application of the residual correction described in § \ref{['residual.sec']}.
• Figure 5: Oversampled slice profiles for cases where the PSF peak is centered in a pixel (left panel) vs between pixels (right panel). Green squares represent the original data in a given column; orange points show the oversampled values from simple linear interpolation between the original points. Blue points show the $N=2$ oversampled values from the spline model fit (black curve).