UV VARIABILITY OF IRAS 13224-3809, A RAPIDLY X-RAY VARIABLE FAR-INFRARED LUMINOUS GALAXY

We have detected strong Lyalpha line variability in IRAS 13224-3809. a very luminous IRAS galaxy known to be a strong and rapidly variable s oft X-ray emitter. While a relatively broad Lyalpha component (FWHM ap proximately 5000 km/s) remains essentially constant along three IUE ob servations performed between January and May 1993, the initially stron g narrow core component vanishes completely becoming a strong absorpti on. IRAS 13224-3809 has a deficit of UV emission when compared to Seyf ert 1 galaxies. The UV-X-ray energy distribution suggests that the UV bump frequently found in these galaxies might be present at higher ene rgies. well within the ROSAT band (0. 1 -2.4 keV). The strong far infr ared emission, the X-ray and Lyalpha profile variability, the absence of broad Balmer lines components and the high Halpha/Hbeta ratios coul d be explained assuming the presence of a nucleus that has become ''ac tive'' after a recent merging episode, being surrounded by large amoun ts of gas and dust which would obscure selectively different emitting regions.