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May 17, 2024
COS NEWSLETTERS

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In this STAN we announce the delivery of new time-dependent sensitivity and photometric throughput tables for FUV observations.

New COS/FUV Time-Dependent Sensitivity and Photometric Throughput Tables Delivered

The sensitivity of the COS FUV detector declines with time. The time-dependent sensitivity (TDS) for each mode is modeled as a piecewise linear function of time and is assumed to be a smooth function of wavelength. This TDS is characterized in a TDSTAB reference file. After the initial conversion of counts to fluxes, the fluxes are further modified by a factor that changes with time. The TDS models are derived from observations of flux standards, which are monitored every two months for the FUV. The monitoring program uses the following subset of available cenwaves to track the TDS for both standard modes and blue modes: 1055, 1096, 1222, 1291, and 1327 for the G130M grating; 1577 and 1623 for the G160M grating; and 800, 1105 and 1280 for the G140L grating.

Starting in 2023, the TDS monitoring program showed systematic deviations from the model (see Figure 1) . For the FUVA detector, the decline was about 3%/yr steeper than predicted at the short wavelength end and about 8%/yr steeper at the long wavelength end. For the FUVB detector, the decline was about 6%/yr steeper than predicted at the short wavelength end and about 10%/yr steeper at the long wavelength end. To correct these deviations, new parameters for the standard modes were derived. The model required a new temporal breakpoint at 2023.2 to account for changes in the slope, likely driven by the increase in solar activity since that time.

The new TDS parameters were used to update the sensitivities at the reference time at lifetime positions (LP) 2, 3, 4, 5, and 6, so the corresponding FLUXTAB files were also updated. A new FUV TDSTAB and new FUV FLUXTABs for LP2, LP3, LP4, LP5, and LP6 were delivered on 2024 March 19. Users who have obtained FUV spectra since March 1st, 2023 are encouraged to retrieve their newly recalibrated data from the HST archive.

 

G130M/1222 monitor observations of WD0308-565. Mean residuals against model spectra over time for a 20-Angstrom wavelength bin of the FUVB (top) and FUVA (bottom).
Figure 1: G130M/1222 monitor observations of WD0308-565. Mean residuals against model spectra over time for a 20-Angstrom wavelength bin of the FUVB (top) and FUVA (bottom). The residuals of the original TDSTAB (black circles) began declining in early 2023 due to rapid sensitivity loss. From 2023.2 onward, a new slope was implemented to account for this loss (blue circles), bringing the measurements back within spec.

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