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

TL;DR

This study uses XRISM and NuSTAR observations to analyze the complex nuclear structure of Seyfert-2 galaxy NGC 4388, revealing detailed properties of its Fe K lines, BLR, and absorbing gas.

Contribution

The paper presents a detailed spectral analysis combining XRISM and NuSTAR data, modeling the reflection, fluorescent lines, and absorption features with updated tools and geometric assumptions.

Findings

Fe K$oldsymbol{ m extalpha}$ line components correspond to different nuclear regions.

The BLR has a larger radius than the H$oldsymbol{ m extalpha}$ line region, indicating equatorial velocity dominance.

Detected absorption lines suggest a gravitationally bound, possibly failed wind, consistent with a fountain flow model.

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 $\sim 290\ \mathrm{km\ s^{-1}}$, $\sim 1470\ \mathrm{km\ s^{-1}}$, and $\sim 11100\ \mathrm{km\ s^{-1}}$. These line widths correspond to radii of 1.5 pc, 0.060 pc, and $1.0\times10^{-3}$ 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 $\mathrm{km\ s^{-1}}$) 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 $\log{\xi} \sim 3.50~\mathrm{erg\ cm\ s^{-1}}$, $\log{N_{\mathrm{H}}} \sim 22.1~\mathrm{cm^{-2}}$, $v_{\mathrm{out}} \sim 40\ \mathrm{km\ s^{-1}}$, and $\sigma_v \sim 160\ \mathrm{km\ s^{-1}}$. We infer that the absorber is gravitationally bound and is possibly associated with a failed wind, consistent with a radiation-driven fountain flow.