AT2022zod: Short, Off-Nuclear TDE

• AT2022zod is an extreme tidal disruption event (TDE) observed off-nuclear, suggesting disruption by an intermediate-mass black hole in a UCD or stripped nucleus.
• The event features a rapid ~30-day light curve with a Gaussian rise and exponential decay, characterized by steep early-time slopes and high peak luminosities.
• Photometric fits using TiDEpy and MOSFiT models indicate a black hole mass in the 10^5–10^6 M☉ range, distinguishing it from TDEs powered by central SMBHs.

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 (Dage et al., 1 Dec 2025).

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)	$~30$0	$~30$1
Peak epoch $~30$2 (MJD)	$~30$3	$~30$4
Rise timescale $~30$5 (days)	$~30$6	$~30$7
Decay timescale $~30$8 (days)	$~30$9	$z=0.11$0
Total duration $z=0.11$1 (d)	$z=0.11$2	$z=0.11$3

Peak apparent magnitudes reached $z=0.11$4 and $z=0.11$5; corresponding peak absolute magnitudes are $z=0.11$6 and $z=0.11$7 (neglecting $z=0.11$8-correction). The rise and decay rates were $z=0.11$9 mag day$D_L = 526.9$0 and $D_L = 526.9$1 mag day$D_L = 526.9$2, 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 $D_L = 526.9$3 (corresponding to $D_L = 526.9$4 kpc and conservatively up to $D_L = 526.9$5 kpc) from the galaxy's photometric centroid, establishing the event as significantly off-nuclear. The host is a quiescent elliptical of stellar mass $D_L = 526.9$6, and the stellar velocity dispersion ($D_L = 526.9$7 km s$D_L = 526.9$8) implies a central SMBH mass of $D_L = 526.9$9.

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:

$g$0

• Fallback accretion rate at late times:

$g$1

• Characteristic fallback time:

$g$2

AT2022zod's $g$3-day duration is significantly shorter than the fallback times expected for disruption by a $g$4 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 $g$5) yields $g$6. An independent fit using the {\tt MOSFiT} reprocessing-layer model gives $g$7. Both approaches strongly favor a MBH in the $g$8–$g$9 mass regime—distinctly below the SMBH mass of the host galaxy center (Dage et al., 1 Dec 2025).

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 $r$0 SMBH, fallback time could be somewhat reduced, yet a 30-day event remains anomalously brief relative to the expected $r$1 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 $r$2–$r$3. 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 (Dage et al., 1 Dec 2025).

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 ($r$4–$r$5), lying at the extreme short-timescale/faint-end compared to the ZTF-I sample. The rise and decay timescales ($r$6 d rise, $r$7 d decay) are among the fastest recorded, with light-curve slopes at early times ($r$8–$r$9, fitted with a $r_0$0 power law) notably steeper than the more typical $r_0$1 (Dage et al., 1 Dec 2025).

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 $r_0$2–$r_0$3\% of optical TDEs may originate in such environments. As future wide-field time-domain surveys come online, particularly the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), search and follow-up strategies must be refined to efficiently identify short-duration ($r_0$4 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 (Dage et al., 1 Dec 2025).

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.