We report evidence for persistent giant and rapid X-ray variability in the radio-quiet, ultrasoft, strong Fe ii, narrow-line Seyfert 1 galaxy IRAS 13224-3809. Within a 30 day ROSAT High Resolution Imager (HRI) monitoring observation at least five giant amplitude count rate variations are visible, with the maximum observed amplitude of variability being about a factor of 60. We detect a rise by a factor of about 57 in just two days. IRAS 13224-3809 appears to be the most X-ray variable Seyfert known, and its variability is probably nonlinear. We carefully check the identification of the highly variable X-ray source with the distant galaxy, and it appears to be secure. We examine possible explanations for the giant variability. Unusually strong relativistic effects and partial covering by occulting structures on an accretion disc can provide plausible explanations of the X-ray data, and we explore these two scenarios. Relativistic boosting effects may be relevant to understanding the strong X-ray variability of some steep spectrum Seyferts more generally.

Redshift surveys of the extragalactic IRAS sources have revealed a class of galaxies, ultraluminous infrared galaxies (ULIRGs), that radiate an enormous amount of energy in the far-infrared (Soifer et al. 1986), typically LIR ≥ 1012L⊙, which is comparable to the luminosities of quasars. The energy source for most ULIRGs is believed to be massive nuclear starbursts triggered by galaxy interactions or mergers (Rigopoulou et al. 1996, Crawford et al. 1996). There is much debate about the evolution of ULIRGs. Sanders et al. (1988) suggested that ULIRGs are forming QSOs. However, it is not clear how merging galaxies evolve to QSOs. Meanwhile, observations and N-body simulations show that mergers between disk galaxies can form ellipticals. Some ULIRGs, such as Arp 220 and NGC 6240, do have elliptical characteristics. What is the evolutionary connection between QSOs and elliptical galaxies? About 10% of ULIRGs are IR QSOs or Seyfert 1 galaxies, on the basis of their optical spectra and appearance. Some of them are hosted in ellipticals (Hutchings et al. 1988, Lipari et al. 1994). Also, most of them are strong Fen emitters. Studying this subsample of ULIRGs in detail may give some clues to understanding the evolutionary process mentioned above.