X-Ray Spectral Variability of the Seyfert Galaxy NGC 4051 Observed with Suzaku

TL;DR

This study analyzes Suzaku X-ray observations of NGC 4051, revealing spectral variability linked to flux changes, with significant reflection and complex absorption features, advancing understanding of Seyfert galaxy X-ray behavior.

Contribution

First detailed Suzaku spectral analysis of NGC 4051, identifying the causes of spectral variability through modeling of reflection and absorption components.

Findings

Spectral hardening observed at lower flux levels.

Large reflection component with R ~ 7 detected.

Spectral variability explained by partial covering and reflection changes.

Abstract

We report results from a Suzaku observation of the narrow-line Seyfert 1 NGC 4051. During our observation, large amplitude rapid variability is seen and the averaged 2--10 keV flux is 8.1x10^-12 erg s^-1 cm^-2, which is several times lower than the historical average. The X-ray spectrum hardens when the source flux becomes lower, confirming the trend of spectral variability known for many Seyfert 1 galaxies. The broad-band averaged spectrum and spectra in high and low flux intervals are analyzed. The spectra are first fitted with a model consisting of a power-law component, a reflection continuum originating in cold matter, a blackbody component, two zones of ionized absorber, and several Gaussian emission lines. The amount of reflection is rather large (R ~ 7, where R=1 corresponds to reflection by an infinite slab), while the equivalent width of the Fe-K line at 6.4 keV is modest (140 eV) for the averaged spectrum. We then model the overall spectra by introducing partial covering for the power-law component and reflection continuum independently. The column density for the former is 1x10^23 cm^-2, while it is fixed at 1x10^24 cm-2 for the latter. By comparing the spectra in different flux states, we identify the causes of spectral variability. (abridged)