---
title: 'AT2022zod: Short, Off-Nuclear TDE'
url: https://www.emergentmind.com/topics/at2022zod
type: topic
---

# AT2022zod: Short, Off-Nuclear TDE

AT2022zod is an extreme and short-lived optical transient observed in an elliptical galaxy at redshift $z=0.11$, interpreted as a Tidal Disruption Event (TDE) most likely caused by an intermediate-mass black hole (IMBH) embedded in an ultra-compact dwarf galaxy (UCD) or a stripped nuclear star cluster. The event is characterized by its rapid $~30$-day timescale, high luminosity for its short duration, and a significant positional offset from the host galaxy's center, distinguishing it from typical TDEs associated with supermassive black holes (SMBHs) in galaxy nuclei [2512.02136].

## 1. Observational Properties

AT2022zod was detected in the elliptical host SDSS J105602.80+561214.7 at $z=0.11$, corresponding to a luminosity distance $D_L = 526.9$ Mpc. The light curve was captured by the Zwicky Transient Facility (ZTF) in both $g$ and $r$ bands and is well described by a Gaussian rise plus exponential decay model.

| Parameter                     | ZTF–g                   | ZTF–r                    |
|-------------------------------|-------------------------|--------------------------|
| Baseline $r_0$ (mag)          | $18.962\pm0.010$        | $17.865\pm0.008$         |
| Flare amplitude $A$ (mag)     | $0.880\pm0.079$         | $0.329\pm0.046$          |
| Peak epoch $t_0$ (MJD)        | $59881.996\pm2.124$     | $59871.344\pm5.113$      |
| Rise timescale $t_g$ (days)   | $13.24\pm3.68$          | $3.93\pm3.87$            |
| Decay timescale $t_e$ (days)  | $15.44\pm2.38$          | $57.06\pm11.07$          |
| Total duration $t_g+t_e$ (d)  | $28.7\pm6.1$            | $61.0\pm18.9$            |

Peak apparent magnitudes reached $g_{\rm peak}\approx18.08$ and $r_{\rm peak}\approx17.54$; corresponding peak absolute magnitudes are $M_{g,\rm peak}\approx-20.5$ and $M_{r,\rm peak}\approx-20.4$ (neglecting $K$-correction). The rise and decay rates were $\sim0.66$ mag day$^{-1}$ and $\sim0.057$ mag day$^{-1}$, respectively.

Spectroscopic observations of the host from SDSS show no strong emission lines; Balmer and forbidden lines are detected at very low signal-to-noise, indicating a "retired"/LINER-like emission region. There were no spectra of the flare itself, thus no transient broad lines were detected.

Astrometric comparison places the transient $<0.7''$ (corresponding to $<1.4$ kpc and conservatively up to $2.4$ kpc) from the galaxy's photometric centroid, establishing the event as significantly off-nuclear. The host is a quiescent elliptical of stellar mass $\log_{10}(M_\star/M_\odot)\approx10.93$, and the stellar velocity dispersion ($\sigma_\star=158.1\pm11.7$ km s$^{-1}$) implies a central SMBH mass of $\sim10^8\,M_\odot$.

## 2. Tidal Disruption Event Framework

A TDE occurs when a star passes sufficiently close to a black hole to be disrupted by tidal forces, producing a luminous flare as stellar debris is accreted. Key scaling relations for classic TDEs include:

- Tidal radius:
  $$
  R_{\rm t} \approx R_\star \left( \frac{M_{\rm BH}}{M_\star} \right)^{1/3}
  $$
- Fallback accretion rate at late times:
  $$
  \dot M(t) \propto t^{-5/3}
  $$
- Characteristic fallback time:
  $$
  t_{\rm fb} \approx 0.1\,\mathrm{yr} \left( \frac{M_{\rm BH}}{10^6M_\odot} \right)^{1/2} \left( \frac{R_\star}{R_\odot} \right)^{3/2} \left( \frac{M_\star}{M_\odot} \right)^{-1}
  $$

AT2022zod's $\sim30$-day duration is significantly shorter than the fallback times expected for disruption by a $\sim10^8\,M_\odot$ SMBH, even allowing for non-parabolic stellar orbits.

## 3. Inferred Black Hole Properties

Fitting the photometric light curve with the {\tt TiDEpy} semi-analytic wind model (assuming polytrope index $\gamma=4/3$) yields $M_{\rm BH} \simeq 5.4 \times 10^5\,M_\odot$. An independent fit using the {\tt MOSFiT} reprocessing-layer model gives $M_{\rm BH} = (1.3^{+0.8}_{-0.5}) \times 10^6\,M_\odot$. Both approaches strongly favor a MBH in the $10^5$–$10^6\,M_\odot$ mass regime—distinctly below the SMBH mass of the host galaxy center [2512.02136].

## 4. Alternative Origin Scenarios

Two primary scenarios are examined for AT2022zod’s origin:

- **Partial-Orbit TDE by the Central SMBH:** For a star on a bound non-parabolic orbit around the $10^8\,M_\odot$ SMBH, fallback time could be somewhat reduced, yet a 30-day event remains anomalously brief relative to the expected $\gtrsim1$ yr timescale for such SMBH disruptions. Additionally, the observed luminosity evolution is inconsistent with Eddington-limited accretion at such high SMBH mass.

- **IMBH in an Off-Nuclear UCD/Stripped Nucleus (favored):** The observed location offset, event duration, and peak luminosity are naturally explained by a TDE powered by a MBH in a UCD with $M_{\rm BH}\sim10^5$–$10^6\,M_\odot$. This interpretation is consistent with the lack of persistent AGN emission lines, the absence of flare recurrence in five years of ZTF monitoring, and no radio or X-ray AGN signatures [2512.02136].

A plausible implication is that AT2022zod demonstrates TDEs as effective probes of off-nuclear IMBHs in UCDs or stripped galaxy nuclei.

## 5. Comparison with Transient and TDE Populations

AT2022zod exhibits one of the shortest and faintest TDE-like light curves in high-mass host galaxies ($M_\star=10^{10.5}$–$10^{11.5}\,M_\odot$), lying at the extreme short-timescale/faint-end compared to the ZTF-I sample. The rise and decay timescales ($\sim13$ d rise, $\sim15$ d decay) are among the fastest recorded, with light-curve slopes at early times ($\sim1.7$–$2.5$, fitted with a $t^{-5/3}$ power law) notably steeper than the more typical $\sim1.5$ [2512.02136].

## 6. Implications for Black Hole Demographics and Future Surveys

AT2022zod supports the hypothesis that some optical TDEs arise from off-nuclear IMBHs in UCDs or stripped nuclei. Simulations (e.g., González Prieto et al. 2025) predict that $0.1$–$10$\% of optical TDEs may originate in such environments. As future wide-field time-domain surveys come online, particularly the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), search and follow-up strategies must be refined to efficiently identify short-duration ($<30$ d), off-nuclear TDEs. Rapid, host-agnostic flare detection algorithms and early multi-band photometry, supplemented by low-resolution spectroscopy to exclude SNe and kilonovae, will be critical for isolating IMBH TDE candidates [2512.02136].

A plausible implication is that AT2022zod will serve as a reference event for optimizing LSST discovery pipelines targeting IMBH demographic studies via optical TDE signatures.

Source: https://www.emergentmind.com/topics/at2022zod