Nature of the Warm Absorber Outflow in NGC 4051

The Narrow-line Seyfert 1 galaxy NGC 4051 is known to exhibit significant X-ray spectral/flux variations and have a number of emission/absorption features. X-ray observations have revealed that these absorption features are blueshifted, which indicates that NGC 4051 has warm absorber outflow. In order to constrain physical parameters of the warm absorber outflow, we analyze the archival data with the longest exposure taken by XMM-Newton in 2009. We calculate the root-mean-square (RMS) spectra with the grating spectral resolution for the first time. The RMS spectra have a sharp peak and several dips, which can be explained by variable absorption features and non-variable emission lines; a lower-ionized warm absorber (WA1: $\log\xi = 1.5,\, v=-650\,\mathrm{km\,s}^{-1}$) shows large variability, whereas higher-ionized warm absorbers (WA2: $\log\xi= 2.5,\,v=-4100\,\mathrm{km\,s}^{-1}$, WA3: $\log\xi= 3.4,\, v=-6100\,\mathrm{km\,s}^{-1}$) show little variability. WA1 shows the maximum variability at a timescale of $\sim10^4$~s, suggesting that the absorber locates at $\sim10^3$ times of the Schwarzschild radius. The depth of the absorption features due to WA1 and the observed soft X-ray flux are anti-correlated in several observational sequences, which can be explained by variation of partial covering fraction of the double-layer blobs that are composed of the Compton-thick core and the ionized layer ($=$WA1). WA2 and WA3 show little variability and presumably extend uniformly in the line of sight. The present result shows that NGC 4051 has two types of the warm absorber outflows; the static, high-ionized and extended line-driven disk winds, and the variable, low-ionized and clumpy double-layer blobs.