Complex Nuclear Structure in Seyfert 2 Galaxy NGC 4388 Revealed by XRISM Observation

Abstract

We report results from the simultaneous XRISM (183 ks) and NuSTAR (62 ks) observations of the Seyfert-2 galaxy NGC 4388. This AGN has the brightest Fe K$\alpha$ line among Compton-thin, obscured sources. To model the reflection continuum and fluorescent lines, we employ an updated version of XCLUMPY and a broad line region model with a disk-like geometry. The profile of the neutral Fe-K fluorescent line is well described as the sum of three components convolved with Gaussians with FWHM values of $\sim 290\mathrm{km}{\mathrm{s}}^{-1}$, $\sim 1470\mathrm{km}{\mathrm{s}}^{-1}$, and $\sim 11100\mathrm{km}{\mathrm{s}}^{-1}$. These line widths correspond to radii of 1.5 pc, 0.060 pc, and $1.0\times 10^{-3}$ pc by assuming Keplerian motion, which we interpret as the dusty torus, its inner edge region, and the BLR, respectively. The data suggest that the Fe K$\alpha$ BLR component is larger than that of H$\alpha$ (FWHM of 4500 $\mathrm{km}{\mathrm{s}}^{-1}$) in the polarized optical spectrum, implying that the velocity field of the BLR is dominated by that parallel to the equatorial plane. In addition, Fe XXVI Ly$\alpha$ and Fe XXV absorption lines are detected, characterized by $\log \xi \sim 3.50\mathrm{erg}\mathrm{cm}{\mathrm{s}}^{-1}$, $\log N_{\mathrm{H}}\sim 22.1\mathrm{c}{\mathrm{m}}^{-2}$, $v_{\mathrm{out}}\sim 40\mathrm{km}{\mathrm{s}}^{-1}$, and ${\sigma }_v\sim 160\mathrm{km}{\mathrm{s}}^{-1}$. We infer that the absorber is gravitationally bound and is possibly associated with a failed wind, consistent with a radiation-driven fountain flow.