X-Ray spectral variability of the seyfert galaxy NGC 4051 observed with Suzaku

Abstract

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 × 10-12ergs-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 ̃ 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 (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 × 10 23 cm-2, while it is fixed at 1 x 1024cm -2 for the latter. By comparing the spectra in different flux states, we could identify the causes of spectral variability. © 2009. Astronomical Society of Japan.