ROSAT monitoring of persistent giant and rapid variability in the narrow-line Seyfert 1 galaxy IRAS 13224-3809

We report evidence for persistent giant and rapid X-ray variability in theradio-quiet, ultrasoft, strong Fe II, narrow-line Seyfert 1 galaxy IRAS13224-3809. Within a 30 day ROSAT High Resolution Imager (HRI) monitoringobservation 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 13224-3809appears to be the most X-ray variable Seyfert known, and its variability isprobably nonlinear. We carefully check the identification of the highlyvariable X-ray source with the distant galaxy, and it appears to be secure. Weexamine possible explanations for the giant variability. Unusually strongrelativistic effects and partial covering by occulting structures on anaccretion disc can provide plausible explanations of the X-ray data, and weexplore these two scenarios. Relativistic boosting effects may be relevant tounderstanding the strong X-ray variability of some steep spectrum Seyferts moregenerally.Comment: 14 pages, submitted to MNRA