XRISM finds the Changing-Look AGN NGC 1365 in an extended low state: A dense, highly ionized outflow obscures the central source

TL;DR

This study uses XRISM/Resolve to observe the Changing-Look AGN NGC 1365 in a persistent low-flux, heavily obscured state, revealing both blueshifted Fe XXV/XXVI absorption and, for the first time in this source, emission lines from the highly ionized wind. Spectral modeling with a photoionized wind (and a competing collisional-emission scenario) shows a wind with $v_{ ext{out}} \,\approx\,-2600$ km s$^{-1}$ and $\,\log\xi\approx3.8$, launched at $R\sim(1.9$–$3.7)\times10^{4} R_g$ (≈ $1.5$–$2.4\times10^{16}$ cm), consistent with BLR scales and co-spatiality with the broad Fe Kα emission. The Fe Kα line is resolved with a broad component ($\sigma\sim1300$ km s$^{-1}$) and disk-broadening fits place the emitting region at $\sim10^{4} R_g$, aligning with optical/IR BLR gas; the broad line shows hints of variability and a shared origin with the wind. The results imply a compact, ionized outflow interacting with circumnuclear gas and support a BLR-scale wind–Fe Kα connection, while indicating only modest feedback power relative to the bolometric luminosity. XRISM demonstrates its capability to dissect obscured AGN physics, motivating future time-resolved, broadband spectroscopy across high- and low-flux states.

Abstract

We present the first XRISM/Resolve observations of the active galactic nucleus, NGC 1365, obtained in 2024 February and July. NGC 1365 is known for rapid transitions between Compton-thick and Compton-thin states, along with strong absorption from a highly ionized wind. During our observations, the source is found in a persistent low-flux state, characterized by a decrease in hard-X-ray luminosity and significant line-of-sight obscuration. In this state, XRISM/Resolve reveals clear Fe\,\textsc{xxv} and Fe\,\textsc{xxvi} absorption lines together with, for the first time in this source, corresponding emission lines. These features may arise either from reemission from a photoionized wind (P Cygni profile) or from collisionally ionized gas associated with outflow-driven shocks in the interstellar medium. We estimate the wind launch radius to be approximately $10^{16}~\mathrm{cm}$ ($\sim 10^4 R_{\mathrm{g}}$), consistent with the location of the X-ray broad-line region. We also resolve a broadened Fe K$α$ line by $σ\sim 1300$ km s$^{-1}$ placing it at similar scales to the wind, consistent with radii inferred from disk-broadening models and the variability of the Fe K$α$ broad line. The similarity of the Fe K$α$ profile to the H$β$ wing and broad Pa$α$ width indicates that the X-ray-emitting region is likely cospatial with the optical/IR broad-line region and originates from the same gas.

Figures (9)

• Figure 1: XMM-Newton EPIC-pn spectra of NGC 1365 from four epochs: 2004 (black), 2007 (green), 2013 (red), and 2024 (blue). The spectra illustrate long-term variability in the 0.3–10 keV band, with transitions between Compton-thin (e.g., 2013) and Compton-thick states (e.g., 2007, 2024). The source also undergoes transitions between these states on much shorter timescales of hours to days. The 2024 observation is contemporaneous with the first XRISM observation (Obs1) and captures the source in a heavily obscured state. All spectra are shown in the observed frame and have been binned for clarity.
• Figure 2: XMM-Newton EPIC-pn light curves of NGC 1365, simultaneous to XRISM/Resolve Obs1, in two energy bands: 2--4 keV (top, green) and 4--10 keV (middle, blue), and the corresponding hardness ratio (bottom, red) defined as the ratio of the 4--10 keV to 2--4 keV count rates. The light curves are binned at 1500 s. The hard band shows up to $\sim50\%$ variability over a $\sim$100 ks observation, suggesting the presence of a transmitted intrinsic continuum component even during this Compton-thick state.
• Figure 3: Long-term X-ray light curves of NGC 1365. The top panel shows the Swift BAT 15--50 keV count rates averaged over $10^8$ s bins. The bottom panel displays the X-ray ($\sim$ 0.3--10 keV) fluxes from XMM-Newton (orange), Swift XRT (black), and NICER (magenta). Vertical dashed lines mark the two XRISM observations (blue & sky blue), as well as archival XMM-Newton observations from 2007 (green) and 2013 (red), corresponding to the spectra shown in Fig. \ref{['fig:XMM']}. The right-hand panel shows a zoom-in of the NICER and Swift monitoring around the time of the XRISM observations. Together, these light curves capture the significant variability of the source and reveal a long-term decline in both soft and hard X-ray flux starting around 57500 MJD, leading to a persistent low-flux state.
• Figure 4: XRISM/Resolve spectra of NGC 1365 in the 6.0--8.0 keV energy range from two epochs: February 2024 (black) and July 2024 (blue). The vertical dashed lines indicate the expected rest-frame energies of key emission and absorption features, including the Fe K$\alpha$ fluorescence line at 6.4 keV, Fe xxv lines at 6.70--6.75 keV, Fe xxvi Ly$\alpha$ lines around 6.95--7.05 keV, and the Fe K$\beta$ and K-edge features near 7.1 keV. Outside this energy range, the spectra are largely featureless aside from known non--X-ray background (NXB) lines. The two epochs show no striking visual differences, and the data have been binned for clarity.
• Figure 5: Comparison of the Fe K$\alpha$ emission region in NGC 1365 as observed by XRISM/Resolve (dark blue, left axis) and Chandra/HETG (magenta, right axis), each with an effective exposure of $\sim$ 450 ks. The Fe K$\alpha$ fluorescence line at 6.4 keV is more clearly resolved in the XRISM data, revealing evidence for intrinsic broadening. This comparison demonstrates XRISM’s enhanced spectral sensitivity and resolution for NGC 1365, even during a relatively faint state of the source.