A 12–day Asca Observation of the Narrow-line Seyfert 1 Galaxy Ton S180: Time-selected Spectroscopy
نویسنده
چکیده
We present an analysis of the X-ray variability properties of the narrow-line Seyfert 1 galaxy Ton S180, based upon 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 < 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 Kα emission is detected. There is also a statistically significant evidence for a narrow Fe Kα line at ∼ 6.8 keV, indicating an origin in ionized material. We do not detect significant variations of the Fe Kα line flux or equivalent width on timescales of ∼ 1 day–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 ∼ 1000 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 ∼ 1 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 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 Γ–soft hump correlation is consistent with the soft hump being produced by up-scattering of the accretion disk radiation within a patchy, flaring disk corona. Subject headings: galaxies: active – galaxies: individual (Ton S180) – galaxies: nuclei – galaxies: Seyfert – X-rays: galaxies
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