Ultraviolet variability of the narrow-line Seyfert 1 IRAS 13224-3809

We present an analysis of the IUE observations of the Narrow-line Seyfert 1 galaxy IRAS13224-3809. This galaxy is a radio-quiet object, very luminous in the IRAS bands and with a very steep (soft) and rapidly variable spectrum in the soft X-ray domain. More remarkably, it has recently revealed giant soft X-ray variations, becoming up to date the most X-ray variable Seyfert known (Boller et al. 1997). We have performed a detailed analysis of the variability behaviour of the whole IUE data set. Only the Ly alpha lambda 1216 core and the continuum flux at wavelengths longer than 1750 Angstrom are found to vary significantly. The largest 1800-Angstrom continuum change (maximum to minimum flux) is close to a factor two but only at the 2.9 sigma level. During the giant amplitude soft X-ray fluctuations mentioned above, and despite of the extreme X-ray behaviour, the available ultraviolet data only suggest a quiescent phase. We have detected Ly alpha and CIV lambda 1549 emission, with complex profiles, showing absorption features and broad wings (not detected in the optical permitted lines of Hydrogen). The continuum spectral energy distribution is similar to that of normal Seyfert Is from the radio to the IR domain and in the hard X-rays. However, it is peculiar in that the UV flux is underluminous compared to the IR or to the X-ray fluxes and in that the soft X-ray spectrum is very steep. It is suggested that the ionizing continuum is extremely hard and intense in IRAS 13224-3809 giving rise to a highly ionized broad-line region where only the UV lines are emitted.