FUSE Photometric Calibration

• The instrument sensitivity as a function of wavelength (detected counts per second per incident erg/cm²/sec/Ångström) will be determined from in-orbit observations of the following hot DA white dwarfs: G191-B2B, HZ 43, GD 153, GD 71, GD 246, GD 659

• The first 4 of these stars are HUT and HST standards, and provide calibrations consistent within 2%-3%.

• The uncertainties in the effective temperatures for these stars may result in significant uncertainties in the model fluxes for any one star in the broad Lyman lines and in the continuum flux shortward of 1050 Ångström s. The use of multiple stars with a wide range of effective temperatures should eliminate this uncertainty (see plot of Astro-2 spectra and corresponding models, adapted from Kruk et al 1999, ApJS 122, 299).

• Systematic effects in the flux calibration at the Lyman lines will also be checked by means of obsrvations of DB white dwarfs with pure He atmospheres. DB's with effective temperatures below about 24K will have neither HI Lyman lines nor He II Balmer lines, and will have a smooth continuum across the FUSE bandpass.

• Version 3 of the flux calibration (in use in the pipeline up until now) was derived early in the mission before we had any observations of the standard stars listed above. Instead, it was derived from a combination of observations of the DA WD2211-495, and from comparison of some stellar spectra with archival HUT spectra. It appears now that our model for WD2211-495 was computed at the wrong effective temperature, with the result that the effective area was overestimated by about 20% for several channels.

• A preliminary new version of the flux calibration has been derived from an observation of G191-B2B. A test of this calibration is shown in the figures of the spectrum of GD 246, overplotted with the model for this star. This new calibration will be installed in the pipeline shortly.

• The so-called worm feature is apparent in the plots of the GD 246 spectra as discrepancies between the observed and predicted flux at long wavelengths, because it did not appear in exactly the same location in the GD 246 data as in the G191-B2B data used to derive the flux calibration.