The COS FUV detectors convert each ultraviolet photon into a shower of electrons, for which the detector electronics calculate the X and Y coordinates and the total charge, or pulse height. Prolonged exposure to light causes the FUV detectors to become less efficient at this photon-to- electron conversion, a phenomenon called "gain sag." As a result, the peak of the pulse-height distribution slowly decreases. As it approaches the minimum threshold imposed by the PHATAB reference file used by CalCOS (currently, 4), target photons may be rejected as background events. Filtering out target photons for which the pulse height has decreased below the current nominal value of 4 produces absorption-like artifacts in extracted spectra, corresponding to the detector regions that have been more damaged by airglow lines. These artifacts start to be noticeable in the B segment of the FUV detector for data acquired after ~ February 2010.
In order to mitigate this effect we are decreasing the minimum threshold used for pulse height filtering by CalCOS from 4 to 2; the updated PHATAB reference file (ucl1623gl_pha.fits) containing this modification was delivered on December 21st, 2010. Users suspecting that their data has been affected by the issue described above can re-retrieve their data through OTFR.
Lowering the minimum value for pulse height screening to 2 leads to an increase of 7% in the average value of the FUV dark count rate; users for which this increase is a concern can reprocess their data using the PHATAB reference file with the minimum threshold value of 4 (u1t1616ll_pha.fits). Lowering the minimum threshold for pulse height screening to 2, also has a small effect in the overall flux calibration, leading to a 5% overestimation of the calibrated flux. This 5% overestimation of the calibrated flux will be corrected when we update the sensitivity reference files in the near future.