Discovery of Changing-Look behavior in AGN NGC 3822: A long-term multiwavelength study

TL;DR

This study presents a 16-year, multiwavelength investigation of the CL-AGN NGC 3822, combining Swift, XMM-Newton, NuSTAR, HCT, and VLT data to trace variability and spectral states from X-ray to optical/UV. X-ray modeling with Comptonization (CompTT) reveals a varying corona with $kT_e$ ≈ 50–88 keV and $\tau$ ≈ 0.77–0.97, while a transient, partially covering absorber appears in 2016–2022, consistent with clouds moving through the line of sight. Optical spectroscopy shows the appearance of broad BAL-like lines in 2022 followed by their disappearance by 2024–2025, indicating CL transitions that correlate with the accretion rate rather than obscuration. A 2022 outburst exhibits a TDE-like UV decay $\propto t^{-5/12}$ and a peak Eddington ratio $\lambda_{Edd} \sim 3.8\times10^{-3}$, linking the CL events to dramatic accretion-rate changes with broader implications for CL-AGN physics and accretion dynamics.

Abstract

We present a comprehensive long-term multi-wavelength study of the active galactic nucleus (AGN) NGC 3822, based on 17 years (2008 to 2025) of X-ray, ultraviolet (UV), and optical observations.The dataset includes observations from Swift, XMM-Newton, and NuSTAR, the Very Large Telescope, and the Himalayan Chandra Telescope. Our multiwavelength light curve analysis reveals flux variations across X-ray to optical/UV bands, with an increased variability amplitude at shorter wavelengths. X-ray spectral analysis indicates the presence of intrinsic absorption during the 2016 and 2022 observations; however, this absorption disappeared before and after these epochs. The presence and absence of the absorber are attributed to clouds moving in and out of the line of sight. During the long-term monitoring period, the bolometric luminosity of the source varies between ($1.32-17)\times10^{43}$ erg s$^{-1}$. Optical spectroscopic monitoring reveals changing-look (CL) behaviour in NGC~3822, characterized by the appearance and disappearance of broad emission lines (BELs). These CL transitions are associated with changes in the Eddington ratio rather than changes in the obscuration. The BELs appear only when the Eddington ratio is relatively high ($\sim 3.8\times10^{-3}$) and disappear when it drops to a lower value ($\sim 0.9\times10^{-3}$).

Figures (11)

• Figure 1: Left: Temporal variation of X-ray (0.3--10 keV range), UV, and optical continuum flux obtained from the Swift (XRT & UVOT) and XMM-Newton (EPIC & OM) observations for the years 2008 to 2024. The shaded areas in cyan and grey indicate the observations for the years 2013 and 2022, respectively, during which the source exhibited significantly enhanced flux levels across all bands compared to other epochs. The dotted blue vertical line indicates the epoch when BELs are detected, while the red line marks the epoch when BELs are not detected. Right: Zoomed-in view of the 2022 monitoring campaign, covering the period from March 2022 to December 2022. The X-ray flux is given in the unit of $10^{-12}$$^{-1}$ cm$^{-2}$ erg cm$^{-2}$ s$^{-1}$ and the optical/UV monochromatic flux is in the unit of $10^{-15}$$^{-1}$ cm$^{-2}$ Å$^{-1}$ erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$.
• Figure 2: Fractional variation of the X-ray, UV, and optical continuum bands as a function of wavelength.
• Figure 3: Left panel (1): The spectra from June 2010 XMM-Newton (XMM) observation, fitted with an absorbed power-law model, and panels (A)--(B) represent the corresponding residual plots. Middle and Right panels (2 and 3): The broadband spectra from the January 2016 (XRT4+NU1) and June 2022 (XRT5b+NU2) observations, are fitted with the Cutoffpl/CompTT model. Panels (A)--(D) display respective residual plots. Detailed explanations of the figure are provided in Section \ref{['sec:Model']}
• Figure 4: The confidence interval plot of the high-energy cutoff parameter $\rm E_{cut}$ for the observation XRT4+NU1 and XRT5b+NU2. The plots illustrate that $\rm E_{cut}$ could not be constrained owing to low-quality data.
• Figure 5: Corner plots of spectral parameters from MCMC analysis for the XRT4+NU1 (left) and XRT5b+NU2 (right) using $\tt CompTT$ model. One-dimensional histograms represent the probability distribution. Three vertical lines in the 1D distribution show 16$\%$, 50$\%$, and 90$\%$ quantiles. We used $\tt CORNER.PY$Foreman-Mackey2017 to plot the distributions. The units of $N_{\rm H}$, $\rm kT_{e}$, and Norm are in $\rm 10^{22}cm^{-2}$, keV, and ; cm$^{-2}$ photons keV$^{-1}$ cm$^{-2}$ s$^{-1}$, respectively.