AT2025ulz: SN IIb and Superkilonova Investigation

• AT2025ulz is an optical/near-infrared transient that initially exhibited kilonova-like light curve features before being classified as a Type IIb supernova.
• The event's detailed follow-up revealed rapid blue-to-red color evolution, a secondary rebrightening, and broad hydrogen and helium spectral lines typical of SN IIb evolution.
• AT2025ulz challenges classification by mimicking kilonova signatures, emphasizing the need for extended multiwavelength, multi-messenger observations to distinguish between true kilonovae and supernova impostors.

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 (Hall et al., 27 Oct 2025). The transient was detected at $\sim$3 hr post-GW trigger at $g\approx 21.0$ mag and $r\approx 21.3$ mag (Yang et al., 21 Oct 2025). 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 (Kasliwal et al., 27 Oct 2025). The event’s host, SDSS J155154.16+305409.3, is a moderately massive ($\sim$0), star-forming spiral galaxy at $\sim$1, with an SFR of $\sim$2 and dust extinction $\sim$3 mag, comparable to typical core-collapse and short gamma-ray burst hosts (Hall et al., 27 Oct 2025, Yang et al., 21 Oct 2025). The transient is offset by $\sim$4 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 $\sim$5 d, and a secondary peak at $\sim$6 d with $\sim$7 mag, paralleling Type IIb SN photometric evolution (Kasliwal et al., 27 Oct 2025). Hubble Space Telescope (HST) observations confirmed that the event remained significantly bluer than canonical kilonovae, e.g., $\sim$8 mag at 4.8 d versus $\sim$97 mag for AT2017gfo (Yang et al., 21 Oct 2025).

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 $g\approx 21.0$0Jy at $g\approx 21.0$1 d) (O'Dwyer et al., 6 Apr 2026). 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 $g\approx 21.0$2 at the event’s distance, and rules out a canonical relativistic GRB origin (O'Connor et al., 27 Oct 2025, O'Dwyer et al., 6 Apr 2026).

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 ($g\approx 21.0$3 d) could be described reasonably by either scenario, though the Bayesian model evidence favored a shock-cooling origin by a factor $g\approx 21.0$4 ($g\approx 21.0$5 vs. $g\approx 21.0$6) (Hall et al., 28 Oct 2025). The best-fit kilonova model required $g\approx 21.0$7 and $g\approx 21.0$8, which are incompatible with expectations for low-chirp-mass ($g\approx 21.0$9) BNS events, where typical ejecta masses are $r\approx 21.3$0 (Yang et al., 21 Oct 2025). The shock-cooling fit implied a low-mass ($r\approx 21.3$1), extended envelope ($r\approx 21.3$2 cm) and explosion energy ($r\approx 21.3$3 erg), matching expectations for Type IIb SNe.

After $r\approx 21.3$4 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 (Hall et al., 28 Oct 2025, Hall et al., 27 Oct 2025, Kasliwal et al., 27 Oct 2025).

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 $r\approx 21.3$5 and $r\approx 21.3$6 yields $r\approx 21.3$7, less than the robust value ($r\approx 21.3$8) seen for GW170817/GRB 170817A, but significantly exceeding random chance (Hall et al., 27 Oct 2025). The host galaxy’s redshift ($r\approx 21.3$9) is within $\lesssim 2$0 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% (Kasliwal et al., 27 Oct 2025).

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” (Wu et al., 29 Apr 2026, O'Dwyer et al., 6 Apr 2026, Kasliwal et al., 27 Oct 2025). In this scenario, the outer regions of a collapsar’s neutrino-dominated disk (NDAF) fragment at $\lesssim 2$1, producing multiple $\lesssim 2$2 neutron star or black hole clumps, which migrate inwards and merge hierarchically, exciting GW emission with high orbital eccentricity ($\lesssim 2$3 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 $\lesssim 2$4 can survive until merger in the LIGO/Virgo band ($\lesssim 2$5 Hz) (Wu et al., 29 Apr 2026). 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 (Wu et al., 29 Apr 2026, O'Dwyer et al., 6 Apr 2026).

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 (Hall et al., 28 Oct 2025). Early-time photometry ($\lesssim 2$6 d) can be equally well described by kilonova, shock cooling, or other unusual transients. Only with extended coverage ($\lesssim 2$7 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 (O'Connor et al., 27 Oct 2025, Hall et al., 27 Oct 2025, Hall et al., 28 Oct 2025).

Deep radio and X-ray limits at the level of $\lesssim 2$8Jy and $\lesssim 2$9 erg cm⁻² s⁻¹ play a decisive role in ruling out afterglow models and off-axis jets in the AT2025ulz case (O'Connor et al., 27 Oct 2025, O'Dwyer et al., 6 Apr 2026). 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 (Hall et al., 27 Oct 2025).

Recommendations arising from this case include prioritizing light curves across optical and NIR bands, monitoring out to $g$0 d, and prompt spectroscopy, with a particular emphasis when candidate subsolar-mass GW mergers reside in star-forming galaxies consistent with collapsar hosts (Hall et al., 27 Oct 2025, Kasliwal et al., 27 Oct 2025, Yang et al., 21 Oct 2025).

7. Implications for Future Observations and Theoretical Models

A robust measurement of non-zero eccentricity ($g$1) 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 (Wu et al., 29 Apr 2026). 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:

(Hall et al., 27 Oct 2025, O'Connor et al., 27 Oct 2025, Yang et al., 21 Oct 2025, Kasliwal et al., 27 Oct 2025, Hall et al., 28 Oct 2025, Wu et al., 29 Apr 2026, O'Dwyer et al., 6 Apr 2026)