Spectral Variability and Iron Line Emission in the ASCA Observations of the Seyfert 1 Galaxy NGC 4051

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Abstract
We present the results of an extensive analysis of the ASCA AO2 observation of the Seyfert 1 galaxy NGC 4051. The target exhibits broadband [0.5–10 keV] variability by a factor ∼ 8 on time scales ∼ 104 s, with a typical doubling time ∼ 500 s. The spectrum is characterized by a strong emission excess over the extrapolated power law at energies E ≤ 1 keV. Absorption edges due to ionized oxygen species O VII and O VIII are detected together with an emission-like feature at E ∼ 0.93 keV. The O VII edge undergoes significant variability on a timescale as low as ∼ 104 s, whilst no contemporary variability of the O VIII feature is detected. Typical variability time scales place constraints on the location and the density of the absorbing matter. In the self-consistent hypothesis of a high energy (E ≥ 2.3 keV) power law reflected by an infinite plane-parallel cold slab, a photon index change (ΔΓ = 0.4) has also been observed; a natural explanation can be found in the framework of non-thermal Comptonization models. The iron line is redshifted (centroid energy E ∼ 6.1 keV) and broad (σ > 0.2 keV); multicomponent structure is suggestive of emission from a relativistic accretion disk; however if the disk is not ionized a contribution by a molecular torus or an iron overabundance by a factor ∼ 1.5 are required.
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