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

We report on results from a Suzaku observation of the narrow-line Seyfert 1 NGC 4051. During our observation, large-amplitude rapid variability was seen, and the averaged 2--10keV flux was 8.1×10^-12erg s^-1cm^-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 were analyzed. The spectra were 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 6.4keV is modest (140eV) for the averaged spectrum. We then modeled the overall spectra by introducing partial covering for the power-law component and reflection continuum independently. The column density for the former is 1×10²³cm^-2, while it is fixed at 1×10²⁴cm^-2 for the latter. By comparing the spectra in different flux states, we could identify the causes of spectral variability.