A 12 day ASCA observation of the narrow-line Seyfert 1 galaxy Ton S180:: Time-selected spectroscopy

We present an analysis of the X-ray variability properties of the narrow-line Seyfert 1 galaxy Ton S180, based on a 12 day continuous observation with ASCA. Examination of the light curves reveals flux variations of a factor of 3.5 in the 0.7-1.3 keV band and 3.9 in the 2-10 keV band. Time-resolved spectroscopy, using approximately daily sampling, reveals that the broad "soft hump" component at energies less than 2 keV shows flux variations on timescales as short as 1 day that are well correlated with the photon index and the 2-10 keV band flux. A broad Fe Kalpha emission is detected. There is also statistically significant evidence for a narrow Fe Kalpha line at similar to6.8 keV, indicating an origin in ionized material. We do not detect significant variations of the Fe Kalpha line flux or equivalent width on timescales of similar to1 day to 1 week. Despite evidence for correlated events in the power law and soft hump on timescales of a day, the flux correlations clearly do not exist on all timescales. In particular, the softness ratio reveals spectral variability on timescales as short as similar to1000 s, indicating that the power-law continuum and soft hump fluxes are not well correlated on this timescale. The softness ratio also shows a slow decline across the observation, due to a combination of the different time variability of the power-law continuum and soft hump flux on timescales of similar to1 week. Our timing analysis and time-selected spectroscopy indicate that the X-ray emission originates within 12 Schwarzschild radii. The amplitudes and timescales of the rapid variations that we observed are consistent with those expected within disk-corona models. Furthermore, the observed fast variability of the soft hump rules out an origin of the soft emission in large-scale components, such as circumnuclear starburst. The Gamma-soft hump correlation is consistent with the soft hump being produced by upscattering of the accretion disk radiation within a patchy, flaring disk corona.