RAPID X-RAY VARIABILITY IN THE I ZW 1-CLASS OBJECT IRAS-13224-3809

X-ray variability in the 0.1 - 2.4keV ROSAT energy band with a doublin g timescale of 800 s and a factor of 4 within a few hours has been det ected in the IRAS AGN 13224-3809. The optical spectrum indicates that IRAS 13224-3809 is a narrow-line Seyfert 1 galaxy with strong permitte d Fe II emission, a member of the unusual I Zw 1 class objects. IRAS 1 3224-3809 appears to be one of the most rapidly variable AGN known so far. This is the first time that variability on a timescale smaller th an 1000 s is reported at such high L(0.1 - 2.4 keV) = 3 . 10(44) erg . s-1 X-ray luminosity in Seyfert galaxies. It is also the first report ed X-ray variability in I Zw 1 class objects. The DELTAt = 800 s varia tion indicates that the X-rays come from a compact region of about 17 light minutes in size. Whereas the X-ray emission varies both in the s oft S(0.1 - 0.4keV) and in the hard H(0.4 - 2.4keV) band, the hardness ratio (H-S)/(H+S) remains constant implying that no significant spect ral changes occur during the X-ray flux variation. The observed- steep X-fay spectrum was compared to powerlaw, blackbody and Bremsstrahlung models. Only the Bremsstrahlung model gives an acceptable fit, but it must be rejected on the basis of the variability argument. Emission f rom a standard accretion disk model fits the data. Again, from the obs erved rapid variability we are forced to rule out this model. A scenar io in which a hard X-ray source irradiates the accretion disk which re emits at soft X-ray energies can explain both, the steep X-ray spectru m and the variability. We speculate that rapid X-ray variability could be a common feature in I Zw 1 type objects and that it may be related to the unusual optical spectra of these sources. This may help to sol ve the open problem of the Fe II/H(beta) ratio in active galactic nucl ei.