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 2–10 keV flux was 8.1 $\times$ 10 $^{-12}$ 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 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 ( $R$ $\sim$ 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 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 $\times$ 10 $^{23}$ cm $^{-2}$ , while it is fixed at 1 $\times$ 10 $^{24}$ cm $^{-2}$ for the latter. By comparing the spectra in different flux states, we could identify the causes of spectral variability.