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2. Ultraviolet observations

The UV spectra have been taken with the International Ultraviolet Explorer (IUE) satellite (Boggess et al. 1978a, b, 1989; Fälker et al.\ 1989). We have used all spectra taken between 1978 and 1991 from the IUE Archive, many of these have been taken under the ESA IUE Observatory Programs. In these 13 years (4903 days) 158 spectra were obtained through the large aperture (LAP: tex2html_wrap_inline4505) in low resolution mode (tex2html_wrap_inline4507), 90 with the SWP (Short Wavelength Prime) camera between tex2html_wrap_inline4509, and 68 with the LWR/P (Long Wavelength Redundant/Prime) cameras between tex2html_wrap_inline4511. All were reduced with the most recent version of the IUE Image Processing System (IUESIPS: Clavel 1987). To adjust for shifts in the wavelength scale due to centering uncertainties in the LAP, all spectra were aligned by centering the peak of tex2html_wrap_inline4513 at tex2html_wrap_inline4515 and for tex2html_wrap_inline4517 at tex2html_wrap_inline4519. The LWR spectra were corrected for the sensitivity loss of this camera with time (Clavel et al.\ 1988). On the other hand, because the sensitivity degradation of the SWP and LWP Cameras is small (some 13% over the 13 years) and shows a smooth variation with wavelength above 1200 Å (Sonneborn & Garhart 1987; Bohlin & Grillmair 1988), its effect on the line shape and continuum variation is very small in comparison to the overall variability of F-9 and no correction was applied for it. The IUE Fine-Error-Sensor (FES) allows optical photometry in a 12'' equivalent square aperture with an effective wavelength for AGN-type energy distributions of tex2html_wrap_inline4523. The 58 FES measurements have been corrected for the sensitivity loss with time. Due to FES reference point (the position on the detector where the photometric measurements are made) changes, two different corrections had to be applied: one between 1980 and 1989 (Pérez et al.\ 1991) and another after 1990 (Pérez 1991).

Rather than listing here again the details for the individual spectra, we refer the reader for the observational details to Clavel et al. (1989) and Courvoisier & Paltani (1992). In most cases the IUE spectra were exposed optimally for tex2html_wrap_inline4525\ and tex2html_wrap_inline4527 but those overexposed in the lines (saturation in more than 5 pixels) have not been used for the line decomposition study (SWP 1804, 2215, 4539, 13351, 16559, 16890, 17249, 17446, 17521, 18094, 18891, 21261, 21286, 25825, 26774, 27007, 27008, 27400, 28213, 28307, 28632, 28633, 29545, 29991, 30160, 31176, 34053, 37548, 40179; LWP 2037, 5872, 6201, 6491, 8081, 8562, 9426). Table 1 gives the UV continuum fluxes and FES counts for the IUE data. Whenever there are several spectra and several FES measurements the values in Table 1 give the mean value and the rms error for the fluxes and the FES counts. In the case of a single FES measurement, we have adopted an error of 6%, corresponding to the average of the rms deviations of the individual measurements at the epochs when more than one measurement was available.


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