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Astron. Astrophys. 329, 863-872 (1998)

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2. Observational material

All spectra were down-loaded from the uniform low dispersion archive (ULDA) version 4.0 in the Swiss ULDA national host in Lausanne. This database contains almost all the low dispersion (resolution [FORMULA] 6 Å) ultraviolet (UV) spectra obtained by the IUE satellite from 1978 to the end of 1991 (Courvoisier & Paltani 1992). The spectra of the ULDA database are processed with the IUESIPS reduction software described by Wamsteker et al. (1989) and references therein. We are aware that the superior NEWSIPS reduction software would improve the signal-to-noise ratio of the spectra. However, the IUE final archive (IUEFA) does not yet contain all the spectra used in this work and we considered that the improvement was not worth repeating the reduction for the more than thousand spectra used in this work.

We considered the 18 Seyfert 1-like objects (quasars and Seyfert galaxies) with more than 15 good quality spectra observed through the large aperture ([FORMULA]) of the short wavelength prime (SWP) camera. Each spectrum was viewed separately and we excluded sky and noisy spectra, as well as some spectra with bright spots. The redshift of 3C 273 is so that the C IV [FORMULA] 1549 line coincides with a reseau mark at [FORMULA] 1792-1796 Å, its most spoiled spectra were also excluded.

Because of the IUE wavelength scale uncertainty due to small displacements of the target within the IUE aperture, we realigned all spectra according to the peak of the geocoronal Ly [FORMULA] [FORMULA] 1216 emission fitted by a Gaussian. This was done for all objects except NGC 4151, because the geocoronal emission is contaminated by its small redshifted (z = 0.0033) Ly [FORMULA] line. We choose the geocoronal emission rather than emission-lines from the object to avoid problems with double-peaked profiles and to use an identical procedure for all objects. The dispersion of the shifts that we measured is [FORMULA] 1.5 Å, but shifts up to [FORMULA] 5 Å are observed. This is enough to introduce spurious antisymmetric line profile variations (Türler & Courvoisier 1997). Finally, the spectra were rebinned into 1 Å bins for computational convenience. Throughout this paper, the flux density is expressed in [FORMULA] and the integrated line flux is expressed in [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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