---
title: 'AT2025ulz: SN IIb and Superkilonova Investigation'
url: https://www.emergentmind.com/topics/ztf25abjmnps-at2025ulz
type: topic
---

# AT2025ulz: SN IIb and Superkilonova Investigation

ZTF25abjmnps (AT2025ulz), also referred to as SN 2025ulz, is an optical/near-infrared transient initially discovered as a candidate electromagnetic counterpart to the subthreshold gravitational-wave trigger S250818k. The event drew significant attention for its early-time light curve and color evolution, which closely resembled expectations for a kilonova associated with a binary neutron star (BNS) merger, but subsequent observations and analysis securely classified it as a young, stripped-envelope Type IIb supernova. AT2025ulz's case has galvanized the field’s discussion of “superkilonova” scenarios, collapsar-disk fragmentation, and the challenges of distinguishing true kilonovae from impostors in the era of large-scale gravitational-wave follow-up.

## 1. Discovery and Initial Characterization

AT2025ulz was first identified by the Zwicky Transient Facility (ZTF; ZTF25abjmnps) within the localization volume of S250818k, a gravitational-wave candidate reported by LIGO–Virgo–KAGRA on 2025 August 18 with a false alarm rate of 2.1 yr⁻¹ and a median luminosity distance of $d_L \simeq 400^{+150}_{-120}$ Mpc [2510.23723]. The transient was detected at $\sim$3 hr post-GW trigger at $g\approx 21.0$ mag and $r\approx 21.3$ mag [2510.18854]. Early optical photometry over the first $\lesssim 2$ days revealed a rapid blue-to-red color evolution and fast decline rates — $g$ band faded by $\sim$1 mag/day — initially consistent with kilonova models. Multiple groups reported “kilonova-like” broadband colors analogous to GW170817.

However, by $\sim$5 days, the light curve rebrightened in the red bands, and spectra obtained with Keck/LRIS and Gemini/GMOS showed the emergence of broad P-Cygni H$\alpha$ absorption ($v_{\rm ej}\sim15{,}000$ km s⁻¹), followed by the development of He I and Ca II features. These characteristics are prototypical of a young, stripped-envelope SN IIb [2510.23732]. The event’s host, SDSS J155154.16+305409.3, is a moderately massive ($M_*\sim10^{10}M_\odot$), star-forming spiral galaxy at $z=0.08484\pm0.000006$, with an SFR of $\sim0.5\,M_\odot\,{\rm yr}^{-1}$ and dust extinction $A_V=0.32^{+0.09}_{-0.10}$ mag, comparable to typical core-collapse and short gamma-ray burst hosts [2510.23723, 2510.18854]. The transient is offset by $\approx1.45$ kpc from the host center.

## 2. Multiwavelength Follow-up and Constraints

Comprehensive follow-up was conducted across optical, near-infrared, radio, and X-ray wavelengths. In the optical/NIR, the light curve showed a fast early decline, a subsequent rebrightening after $t\gtrsim5$ d, and a secondary peak at $t\approx12$ d with $M_r\approx-17$ mag, paralleling Type IIb SN photometric evolution [2510.23732]. Hubble Space Telescope (HST) observations confirmed that the event remained significantly bluer than canonical kilonovae, e.g., $F336W-F160W\approx 1.4$ mag at 4.8 d versus $\sim$7 mag for AT2017gfo [2510.18854].

Extensive X-ray (Swift, XMM-Newton, Chandra) and radio (VLA, MeerKAT, uGMRT) monitoring produced deep upper limits and, at late times, the detection of faint but significant radio emission at 6–10 GHz (peak $F_{6\mathrm{GHz}}\sim 14\,\mu$Jy at $t\sim89$ d) [2604.05128]. The radio light curve is consistent with optically thin synchrotron from shock-heated ejecta or, alternatively, non-thermal emission from an off-axis mildly relativistic jet, but no accompanying X-ray afterglow was found. The combined radio/X-ray dataset excludes a GW170817-like afterglow for viewing angles $\theta_v\lesssim12.5^\circ$ at the event’s distance, and rules out a canonical relativistic GRB origin [2510.23728, 2604.05128].

## 3. Physical Nature: SN IIb, Kilonova, or Superkilonova?

Initial confusion arose from the photometric and color behavior of AT2025ulz in the first days, which were well fit by both kilonova and SN shock-cooling models. Model comparisons using fitting codes (“possis”/NMMA for kilonovae; Piro 2021 analytic for shock cooling) found that early data ($t<5$ d) could be described reasonably by either scenario, though the Bayesian model evidence favored a shock-cooling origin by a factor $\mathcal{B}_{\rm SC}^{\rm KN}\sim10^{3.3}$ ($\ln Z_{\rm KN} = -23.90$ vs. $\ln Z_{\rm SC}=-16.24$) [2510.24620]. The best-fit kilonova model required $m_{\rm ej,tot}\approx0.13\,M_\odot$ and $v_{\rm ej,wind}\approx0.05c$, which are incompatible with expectations for low-chirp-mass ($\mathcal{M}_c < 0.87M_\odot$) BNS events, where typical ejecta masses are $\lesssim0.01\,M_\odot$ [2510.18854]. The shock-cooling fit implied a low-mass ($M_e\sim0.12\,M_\odot$), extended envelope ($R_e\sim3\times10^{13}$ cm) and explosion energy ($E_e\sim3\times10^{50}$ erg), matching expectations for Type IIb SNe.

After $t>5$ d, strong divergence from kilonova models became evident: AT2025ulz exhibited a color plateau, secondary peak, and the unambiguous presence of broad hydrogen and helium lines, all features inconsistent with kilonovae. Radioactive nickel heating, not r-process-powered kilonova physics, drove the late-time rebrightening and color evolution [2510.24620, 2510.23723, 2510.23732].

## 4. Association with the GW Source S250818k

The temporal and spatial coincidence between AT2025ulz and S250818k fueled extensive analysis of the likelihood of a genuine association. The overlap integral calculated via $P_{\rm GW}(\Omega,d_L)$ and $P_{\rm EM}(\Omega,d_L)$ yields $\log_{10}\mathcal{I}\sim3.9–4.2$, less than the robust value ($\sim6$) seen for GW170817/GRB 170817A, but significantly exceeding random chance [2510.23723]. The host galaxy’s redshift ($z=0.08484\pm0.000006$) is within $2\sigma$ of the GW-inferred distance. However, the observed Type IIb SN features and post-peak light curve are intrinsically incompatible with BNS kilonova models. The estimated chance-coincidence probability, based on Type IIb SN volumetric rates and GW localization, is 3–5% [2510.23732].

A plausible implication is that, while the spatial and temporal alignment is noteworthy, the physical evidence requires classifying AT2025ulz as an “interloper” — a supernova unrelated to the GW event, albeit in a parameter space that can easily mimic kilonovae in early-time follow-up.

## 5. Superkilonova Hypothesis and Collapsar-Disk Fragmentation

Motivated by the low-chirp-mass nature of S250818k and the theoretically predicted link between disk fragmentation in collapsar environments and the formation/merger of subsolar-mass compact objects, researchers examined whether AT2025ulz could represent a “superkilonova” [2604.26912, 2604.05128, 2510.23732]. In this scenario, the outer regions of a collapsar’s neutrino-dominated disk (NDAF) fragment at $r\gtrsim100\,r_{\rm g}$, producing multiple $m\sim0.02–0.1\,M_\odot$ neutron star or black hole clumps, which migrate inwards and merge hierarchically, exciting GW emission with high orbital eccentricity ($e_0\sim0.6$ initially).

Numerical relativity simulations (using non-spinning puncture BHs in coplanar configurations) reveal that hierarchical mergers in such disks impart velocity kicks that significantly increase binary eccentricity. Simulation results show that, even after GW-driven circularization, residual eccentricity up to $e\simeq0.1–0.5$ can survive until merger in the LIGO/Virgo band ($f_{\rm GW}=100–500$ Hz) [2604.26912]. Moreover, the predicted superkilonova would combine signatures of an ordinary core-collapse SN and a central subsolar-mass GW event. The detection of orbital eccentricity in a subsolar-mass GW inspiral, coincident with an AT2025ulz-like optical transient, would be a distinct signature of hierarchical assembly in a collapsar disk.

While AT2025ulz itself lacks direct evidence for r-process-powered kilonova ejecta (the spectroscopic and light-curve features are dominated by SN physics), the ongoing interest in this channel remains high, as hierarchical disk fragmentation remains one of the physically plausible routes to subsolar-mass compact object mergers [2604.26912, 2604.05128].

## 6. Multi-messenger Follow-up: Methodological Lessons and Prospects

AT2025ulz exemplifies the practical and methodological difficulties of robustly identifying kilonova counterparts to GW triggers, particularly as surveys reach greater depth and distance [2510.24620]. Early-time photometry ($t<5$ d) can be equally well described by kilonova, shock cooling, or other unusual transients. Only with extended coverage ($t>10$ d), multiwavelength follow-up (especially in NIR, X-ray, and radio), and prompt spectroscopy to reveal key features (e.g., broad hydrogen/helium lines for SNe, lanthanide blanketing for kilonovae) can an unambiguous classification be achieved [2510.23728, 2510.23723, 2510.24620].

Deep radio and X-ray limits at the level of $F_{6\,{\rm GHz}}\lesssim9\,\mu$Jy and $F_X\lesssim2\times10^{-15}$ erg cm⁻² s⁻¹ play a decisive role in ruling out afterglow models and off-axis jets in the AT2025ulz case [2510.23728, 2604.05128]. Systematic redshift and host characterization using large-area spectroscopic surveys (DESI) allows rapid contextualization of transient–GW associations and efficient subtraction of host galaxy light from transient spectra, further improving classification performance [2510.23723].

Recommendations arising from this case include prioritizing light curves across optical and NIR bands, monitoring out to $\gtrsim200$ d, and prompt spectroscopy, with a particular emphasis when candidate subsolar-mass GW mergers reside in star-forming galaxies consistent with collapsar hosts [2510.23723, 2510.23732, 2510.18854].

## 7. Implications for Future Observations and Theoretical Models

A robust measurement of non-zero eccentricity ($e\gtrsim0.05$) in any subsolar-mass GW inspiral, coupled with an AT2025ulz-like electromagnetic counterpart, would serve as strong evidence for hierarchical formation via disk fragmentation in collapsars [2604.26912]. Next-generation GW detectors (Cosmic Explorer, Einstein Telescope) will enhance sensitivity to such GW signals, enabling the exploration of much lower masses and residual eccentricities.

In electromagnetic follow-up, wide-field, deep, multi-color optical/NIR imaging and rapid spectroscopic classification are crucial for distinguishing true kilonovae from SN impostors. Radio and X-ray campaigns remain essential for constraining (or detecting) relativistic ejecta and afterglows. This incident underscores the necessity of coordinated, panchromatic, multi-messenger campaigns to probe the full landscape of compact object mergers and their heterogeneous transient signatures.

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**References**:  
[2510.23723], [2510.23728], [2510.18854], [2510.23732], [2510.24620], [2604.26912], [2604.05128]

Source: https://www.emergentmind.com/topics/ztf25abjmnps-at2025ulz