New Time-Dependent WFC3/IR Inverse Sensitivities

TL;DR

This work addresses the need for reliable long-term flux calibration of the WFC3/IR channel by introducing time-dependent inverse sensitivities. The authors compute new inverse sensitivities $S'$ using corrected observed counts $C$ and synthetic fluxes $F$ from updated throughput curves via $S' = C / F$, where $F$ is derived from the CALSPEC SED convolved with the instrument response and the relation $N_e = \frac{A}{hc} \int F_{\lambda} \cdot \lambda \cdot R \, d\lambda$, with $F = S N_e$ and $S = \frac{hc}{A \int \lambda R \, d\lambda}$. They provide AB, Vega, and ST zeropoints at the reference epoch $MJD = 55008$ and deliver updated filter throughputs and an IMPHTTAB to enable time-dependent $PHOTFLAM$/$PHOTFNU$ in image headers, alongside on-the-fly $S'$ calculation via stsynphot. Validation against GD153 and Sgr A* photometry shows internal precision better than 0.5% and improved agreement with expectations when applying the time-dependent corrections, highlighting the practical impact for archival data flux calibration and future observations.

Abstract

We present new time-dependent inverse sensitivities for the WFC3/IR channel. These were calculated using the sensitivity change slopes measured by \citet{2024wfc..rept....6M} and photometry of five CALSPEC standards (the white dwarfs GRW+70~5824, GD~153, GD~71, G191B2B, and the G-type star P330E) collected from 2009 to 2023. The new inverse sensitivities account for losses of 1-2\% over 15 years, depending on wavelength, and provide an internal photometric precision better than 0.5\% for all wide--, medium--, and narrow-band filters. An updated version of \texttt{calwf3} (v3.7.2) has been developed for use with a new time-dependent image photometry table (IMPHTTAB) and will be used to update the image header photometric keywords following MAST reprocessing, expected in late-2024. Alternatively, the new inverse sensitivities may be computed by the user for a specific observation date by running \texttt{stsynphot}.

Figures (7)

• Figure 1: Ratio of the observed to synthetic count rates, calculated as the weighted mean of the five CALSPEC standard stars in the 15 WFC3/IR filters: wide (black circles), medium (red diamonds), and narrow (blue squares). Error bars are displayed and only wide- and medium-band filters are labeled. The dashed black line indicates a unity ratio, and the dotted line shows the mean ratio over all filters.
• Figure 2: Ratio of the observed to synthetic count rates in five wide-band and four medium-band WFC3/IR filters as a function of pivot wavelength for the five CALSPEC standards used in the calibration. The dashed black line indicates a unity ratio. Error bars are displayed.
• Figure 3: Top: Comparison between the new time-dependent and previous ZPs for WFC3/IR in the ST photometric system for the wide- (black circles) and medium-band (red diamonds) filters, calculated for the reference epoch (June 2009), plotted against pivot wavelength. A dotted line indicates a ZP difference of zero, and the dashed/dotted line shows the mean ZP difference over all filters. Error bars are displayed. Bottom: Same comparison but the new time-dependent ZPs were calculated for the current epoch (July 2024).
• Figure 4: New time-dependent PHOTFLAM values extracted from the header of the FLT images of the standard WD GD153 observed over multiple epochs in the F125W filter (filled red diamonds) compared to the constant PHOTFLAM value from the previous photometric calibration (filled black circles).
• Figure 5: Observed relative to synthetic fluxes in the F125W filter for the standard WD GD153 as a function of epoch after calibrating the observations with the time-dependent PHOTFLAM values (filled red diamonds) or with the constant PHOTFLAM value from the previous calibration (filled black circles). The dashed red line and the dotted black line indicate the unity value and the mean one, respectively.