Unraveling the Nature of the Nuclear Transient AT2020adpi

TL;DR

AT2020adpi is a luminous nuclear transient in a z=0.26 post-starburst host that exhibits both TDE-like and AGN-like characteristics. The study implements a comprehensive multiwavelength campaign, including ZTF/GAIA/Swift/WISE photometry and SNIFS/LRIS/KCWI/SpeX spectroscopy, to dissect its emission mechanisms. The SED near peak follows a power law with α=0.44, and a significant mid-infrared dust echo lags the optical by ~$208$ rest-frame days, implying a dust radius of ~0.2 pc; the peak optical luminosity is ~$3.6 imes10^{44}$ erg s^{-1} and MIR peak ~$2.3 imes10^{44}$ erg s^{-1}. Optical spectra show broad Balmer and He lines alongside Fe II complexes and coronal Fe lines, with line widths narrowing as the transient fades, a behavior reminiscent of TDEs but within an AGN-like context. Collectively, AT2020adpi is best described as an ambiguous nuclear transient (ANT), likely powered by accretion triggered by a stellar disruption in a pre-existing AGN, underscoring the need for long-term, multiwavelength monitoring to distinguish extreme AGN variability from TDEs and related phenomena.

Abstract

Transient events associated with supermassive black holes provide rare opportunities to study accretion and the environments of supermassive black holes. We present a multiwavelength study of AT2020adpi (ZTF20acvfraq), a luminous optical/UV transient in the nucleus of the galaxy WISEA J231853.77$-$103505.6 ($z=0.26$) that exhibits the properties of an ambiguous nuclear transient. Near peak, its spectral energy distribution is well described by a power law ($λL_λ\propto λ^{-α}$, $α= 0.44 \pm 0.04$), with a maximum $g$-band luminosity of $(3.6 \pm 0.6)\times10^{44}$ erg s$^{-1}$, which is consistent with luminous AGN flares. We detect a strong mid-infrared flare ($L_\mathrm{peak}^{\mathrm{MIR}} = (2.3 \pm 0.05)\times10^{44}$ erg s$^{-1}$) delayed by $\sim$210 rest-frame days, indicating a hot dust echo from material at $\sim$0.2 pc. The optical and near-infrared spectra show broad H, He I, [OIII] lines, as well as narrow Fe II, and prominent Mg II, which is a combination not typical of TDEs. Taken together, these features suggest AT2020adpi is an ambiguous nuclear transient, where an accretion episode was triggered by stellar disruption of an accretion disk or instabilities within an active nucleus. This source demonstrates the need for careful multiwavelength analysis to distinguish between extreme AGN variability and TDEs.

Figures (8)

• Figure 1: Host-subtracted UV, optical, and IR light curves of AT2020adpi, showing the Swift UVM2 band, ZTF $g$,$r$, $i$ bands, Gaia $G$ band, and WISE $W1$ and $W2$ bands. The photometry spans from roughly 800 days before peak (MJD = 59378.267) to roughly 1500 days after peak in observer-frame days. The gray ZTF points are the 100-day moving average of the pre-event ZTF fluxes. The blue (red) bars along the time axis show the epochs of optical (near-IR) spectroscopic follow-up. The black line is the adopted time reference. All data are corrected for Galactic extinction and are in the AB magnitude system, including the WISE data. The light curves have been offset for visual clarity.
• Figure 2: SNIFS, LRIS, and KCWI optical spectra of AT2020adpi, where the time after peak in the observed frame is to the right of each spectrum. The spectra span from almost 1 day after peak UV/optical emission (top) until 1122 days after peak (bottom). Prominent emission lines are labeled. The green shaded region highlights the Fe$\;$ emission complex. The gray shaded regions show the telluric bands. The spectra are offset for visibility.
• Figure 3: SpeX near-IR Spectra of AT2020adpi, where the time after peak in the observed frame is given above the spectra. Prominent emission lines are labeled. The vertical gray bands mark atmospheric telluric features. Due to an overexposed calibration standard on the night of the first observation, the spectrum taken at 56 days post-peak (green) was re-reduced using a standard from a different night at a similar airmass. This improved the overall spectral shape, but left large residuals in the telluric bands.
• Figure 4: SED of AT2020adpi near peak, 350 days and 1500 days post-peak. The open markers show the SED of the host galaxy. The FAST SED model is shown for the host and power-law fits $\lambda L_\lambda \propto \lambda^{-\alpha}$ for AT2020adpi, with $\alpha=0.44\pm0.04$ near peak and $\alpha=-0.29\pm0.05$ at 350 days post peak. The Eddington luminosity is also indicated on the secondary y-axis, showing that the peak luminosity reaches approximately $15\%$ of the Eddington limit.
• Figure 5: Evolution of the FWHM of Mg$\;$ and H$\alpha$ emission lines in the optical spectra AT2020adpi. The line width scales with the continuum flux, since as the transient fades in time, the line width decreases, which is seen in TDEs. The apparent trend in the LRIS points is just noise.