Updated Sensitivities of the Five STIS L-mode Gratings

Abstract

Re-derivation of the sensitivities of all of the Space Telescope Imaging Spectrograph (STIS) observing modes were required after major updates were introduced to the model atmospheres of the three primary standard stars. The new predicted continuum fluxes were up to 2-3% different from the models used to originally calibrate STIS. This work focuses on the re-derivation of spectral sensitivities for the five STIS low-resolution (L-mode) gratings: G140L, G230L, G230LB, G430L, and G750L, which span wavelengths from the far-ultraviolet through the near infrared. Updated photometric throughput tables were delivered to the Calibration Reference Data System (CRDS) on April 7, 2022 and April 14, 2023, which triggered a recalibration of all historical STIS datasets taken with these modes. The sensitivities derived from each of the standard stars typically agree with one another to better than 1%, though discrepancies as large as 1.5% are found in spectral regions most impacted by hydrogen absorption.

Figures (8)

• Figure 1: A comparison of the CALSPEC version 11 models of the three primary white dwarf standard stars compared to the older models that were used in the previous determination of the STIS L-mode sensitivities. The comparison is made in both the absolute flux (top) and the ratio (bottom).
• Figure 2: The top panel shows the raw NET count rates of individual G230LB observations of the three primary standards. In the second panel, the NET count rates have been multiplied by a correction factor to account for time and temperature sensitivity variations and converted to electrons per second (taking into account the CCD gain). The individual spectra are now shown in gray with the averages overlaid in color. Most of the spread between observations of a given target is gone.
• Figure 3: The sensitivities and residuals for each standard star and the average sensitivity for G140L (top) and G230L (bottom). The spline nodes are overlaid at the bottom of each as arrows.
• Figure 4: Same as \ref{['fig:G140L_sens']} for the 3 CCD gratings.
• Figure 5: Plots showing the ratio of the smoothed average flux of the data set after being re-run with the updated PHOTTAB over the CALSPEC model for each standard star for the MAMA detectors. The standard deviations are shown as the banded regions. Typically, the re-reduced data agree with the model to better than 1% on average, and within 2% for individual datasets. Exceptions include the grating edges, where S/N is lowest, and in the cores of narrow absorption or emission features (e.g., the Lyman $\alpha$ line in G140L).