ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger

TL;DR

This work reports a potential multi-messenger association between a sub-threshold GW trigger S250818k and the optical transient ZTF25abjmnps (AT2025ulz). While kilonova-like optical emission can be fit with two-component ejecta and an off-axis afterglow, the spectroscopic evolution and radio/X-ray limits are inconsistent with a canonical GW170817-like kilonova. The authors also demonstrate that the transient is spectroscopically consistent with a young Type IIb supernova, and they quantify a non-negligible chance coincidence within the GW localization. To reconcile these findings, they propose a broader “superkilonova” framework involving disk fragmentation or core fission in a core-collapse SN, outlining observational and theoretical tests for future events to establish a robust EM-GW association.

Abstract

On August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported gravitational waves from a sub-threshold binary neutron star merger. If astrophysical, this event would have a surprisingly low chirp mass, suggesting that at least one neutron star was below a solar mass. The Zwicky Transient Facility mapped the coarse localization and discovered a transient, ZTF25abjmnps (AT2025ulz), that was spatially and temporally coincident with the gravitational wave trigger. The first week of follow-up suggested properties reminiscent of a GW170817-like kilonova. Subsequent follow-up suggests properties most similar to a young, stripped-envelope, Type IIb supernova. Although we cannot statistically rule out chance coincidence, we undertake due diligence analysis to explore the possible association between ZTF25abjmnps and S250818k. Theoretical models have been proposed wherein sub-solar neutron star(s) may form (and subsequently merge) via accretion disk fragmentation or core fission inside a core-collapse supernova i.e. a ``superkilonova". Here, we qualitatively discuss our multi-wavelength dataset in the context of the superkilonova picture. Future higher significance gravitational wave detections of sub-solar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multi-messenger association.

Figures (8)

• Figure 1: The discovery of ZTF 25abjmnps. Upper Left: Zoomed-in skymap of S250818k, with the 50% and 95% contours marked by grey lines, and the position of ZTF 25abjmnps marked by a white star. The other excluded ZTF candidates are marked by black dots. ZTF 25abjmnps lies within the central 50% region, close to the center. Center: Zoomed-out skymap of S250818k. Lower Left: Line-of-sight distance at the position of ZTF 25abjmnps. The median GW distance (262 Mpc) at the location of ZTF 25abjmnps is illustrated with the vertical dotted black line, and the shaded regions correspond to the $\pm1\sigma$, $\pm2\sigma$ and $\pm3\sigma$ regions. The luminosity distance of ZTF 25abjmnps is illustrated with the dashed red line, and lies within $2\sigma$ of the median value. Right: False-color ZTF discovery image of ZTF 25abjmnps in $g-$band and $r-$band (top), alongside the template reference image (center) and difference image (bottom). Differencing yields a clean PSF-like excess in both filters at the location of the transient.
• Figure 2: Optical and near-infrared lightcurve of ZTF 25abjmnps (AT2025ulz). The filters $g$, $r$, $i$, $z$, $J$,$H$ and $Ks$ are shown with different colors and different symbols. We show with a vertical line the onset of S250818k defined as $t=0$ days, another vertical line for the second g-band peak and the second r-band peak. GP fit to the light curve is shown with the solid colored lines and uncertainty as shaded region.
• Figure 3: Spectroscopic evolution of ZTF 25abjmnps subtracting a scaled archival host spectrum Hall2025desihall_at2025ulz_2025. A blue featureless continuum evolved to become redder and then showed prominent broad P-Cygni features. Hydrogen, Helium and Calcium lines are maked with vertical dashed lines at the velocity indicated in parantheses. The spectral range impacted by telluric correction is shown with a gray shaded line and should be ignored. The flux calibration in the GMOS spectrum is impacted by using a flux calibrator on a different night instead of the same night.
• Figure 4: Left panels: $griz$ light curves of ZTF 25abjmnps (filled circles) compared to GW170817/AT2017gfo (open diamonds). Right panel: Combined kilonova and off-axis afterglow model plotted with observed data. The best-fit kilonova model corresponds to the following parameters: $m_{\rm dyn}=0.02\,M_\odot$, $\bar{v}_{\rm dyn}=0.2$c, $\bar{Y_e}_{\rm dyn}=0.2$, $m_{\rm wind}=0.09\,M_\odot$, $\bar{v}_{\rm wind}=0.03$c, $\bar{Y_e}_{\rm wind}=0.3$, $\theta_{\rm obs}=12^\circ$. The afterglow fit yields $\log_{10}(E_0 /\mathrm{erg}) = 53.88 \pm 0.08$ with external medium density of $\log_{10}(n_0 /\mathrm{cm}^{-3}) = -0.97 \pm 0.09$. The structured jet has initial Lorentz factor of $48.20 \pm 15.11$, with the jet core angle $\theta_c = 1.42\degr \pm 0.12\degr$, and the viewing angle $\theta_{\rm obs} = 12.04\degr \pm 0.21\degr$. The electron energy distribution is characterized by an index of $p = 2.88 \pm 0.08$. Note that for a better constrained fit, we fixed the microphysical parameters to typical afterglow values, adopting $\epsilon_e = 0.1$ and $\epsilon_b = 0.01$.
• Figure 5: Comparing ZTF 25abjmnps with template spectra of SNe IIb, Ib and Ic from 2016liu2016modjaz. The templates were constructed from phase-binned, flattened mean spectra with the shaded regions representing 1$\sigma$ diversity in each sub-class. Observed spectra were extinction corrected and continuum subtracted after masking the telluric features and narrow host emission lines. The matches to H$\alpha$ before maximum (Panels 2, 3 4) and Helium after maximum (Panel 6) and the absence of O I support the spectroscopic classification as a Type IIb supernova.