Spectral Variability and iron line emission in the ASCA Observations of the Seyfert 1 Galaxy NGC4051

Abstract

We present the results of an extensive analysis of the ``ASCA'' AO2 observation of the Seyfert 1 galaxy NGC4051. The target exhibits broadband [0.5--10 keV] variability by a factor $\sim 8$ on time scales $\sim 10^{4}s$, with a typical doubling time $\sim 500s$. The spectrum is characterized by a strong emission excess over the extrapolated power law at energies $E\le 1keV$. Absorption edges due to ionized oxygen species OVII and OVIII are detected together with an emission-like feature at $E\sim 0.93keV$. The OVII edge undergoes significant variability on a timescale as low as $\sim 10^{4}s$, whilst no contemporary variability of the OVIII 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 ($E\ge 2.3keV$) power law reflected by an infinite plane-parallel cold slab, a photon index change ($\Delta \Gamma =0.4$) 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 $E\sim 6.1keV$) and broad ($\sigma >0.2keV$); 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 $\sim 1.5$ are required.

17 pages, latex, 12 figures. To be published in Publication of Astronomical Society of Japan