---
title: 'S251112cm: Subsolar GW Candidate'
url: https://www.emergentmind.com/topics/s251112cm
type: topic
---

# S251112cm: Subsolar GW Candidate

Searching arXiv for recent papers on S251112cm and related sub-solar GW interpretations.
S251112cm is a gravitational-wave candidate reported by the LIGO–Virgo–KAGRA collaboration on 2025 November 12 UT and distinguished by a source-frame chirp mass in the sub-solar regime. Across the current literature, it is discussed as a compact object merger candidate with at least one sub-solar mass component, a luminosity distance of approximately \(93 \pm 27\) Mpc, and no confirmed electromagnetic counterpart. These properties place it outside standard stellar-evolution expectations and have motivated two principal research directions: electromagnetic searches for nonstandard merger counterparts, including kilonova and superkilonova scenarios, and population-level interpretations in terms of primordial black holes [2605.10940] [2603.17009] [2602.21295] [2603.25795].

## 1. Detection, inferred source properties, and observational status

S251112cm was observed on 2025 November 12 15:18:45 UTC by the LIGO–Virgo–KAGRA network. The reported false-alarm rate is \(1\) per \(6.2\) yr, although one concurrent GCN gave \(1\) per \(4\) yr; GraceDB and published maps retained \(6.2\) yr [2603.17009]. A related analysis states that the event was initially reported with false alarm rate \( \mathrm{FAR} \approx 1/6.2\,\mathrm{yr}\), later refined to \(\sim 1\) per \(4\) yr in the MBTA SSM pipeline [2605.10940] [2602.21295]. The log-Bayes coherence factor across the three detectors is reported as \(\ln B = +6.1\), indicating a coherent multi-detector signal [2603.17009].

The luminosity distance is reported as \(D_L = 93 \pm 27\) Mpc [2603.17009] [2602.21295]. The source chirp mass is given in the interval \(0.10\)–\(0.87\,M_\odot\) [2605.10940] [2603.17009] [2602.21295], and the probability that the system contains at least one subsolar-mass object is stated as \(100\%\) or \(>99\%\), depending on the analysis [2603.17009] [2602.21295]. The LVK quantity \(P(\mathrm{HasNS}) = 0.08\) indicates an \(8\%\) probability of containing a neutron star in the \(1\)–\(3\,M_\odot\) range by LVK definition [2603.17009].

The chirp mass is related to component masses by
\[
\mathcal{M}_c = \frac{(m_1 m_2)^{3/5}}{(m_1+m_2)^{1/5}},
\]
with the component masses also expressible in terms of the mass ratio \(q \equiv m_2/m_1 \le 1\) as
\[
m_1 = \mathcal{M}\,\left(\frac{1+q}{q^3}\right)^{1/5},
\qquad
m_2 = q\,m_1 .
\]
One summary notes that the chirp-mass bin \(0.10\)–\(0.87\,M_\odot\) implies component masses \(m_1 \simeq 0.30\,M_\odot\), \(m_2 \simeq 0.30\,M_\odot\) under reasonable priors [2605.10940]. Another concludes that, because \(\mathrm{HasNS}=0.08\) and \(\mathrm{HasSSM}=1.0\), the most likely interpretation is a roughly symmetric binary of two subsolar-mass objects [2603.17009].

Sky-localization estimates differ across analyses. One electromagnetic follow-up study describes a gravitational-wave localization spanning \(\sim 100\,\mathrm{deg}^2\) for the \(50\%\)/\(90\%\) credible regions [2605.10940], whereas another PBH-oriented summary quotes a \(90\%\) area of \(\sim 1681\,\mathrm{deg}^2\) [2602.21295]. This suggests that the literature is using different map products, confidence definitions, or summarized sky-area conventions rather than presenting a single uniform localization number.

## 2. Electromagnetic follow-up campaigns and candidate-vetting frameworks

Several groups conducted rapid electromagnetic follow-up. One campaign began \(2.4\) hr after the alert and used the Dark Energy Camera (DECam), the Fraunhofer Telescope at Wendelstein Observatory (FTW), and the Zwicky Transient Facility (ZTF), jointly covering \(56\%\) of the GW probability, or \(60\%\) including WFST [2605.10940]. The facilities and strategies were distinct: FTW/3KK performed galaxy-targeted observations in \(r,i,J\) bands to DESI DR1 hosts; ZTF executed snapshot tiling of the northern lobe in \(g,r\) at \(\sim 18\) hr post-alert; and DECam surveyed the southern lobe in \(g,i\) from \(1.4\) d post-event [2605.10940].

A separate counterpart search combined triggered observations with SAGUARO telescopes, community alerts, and survey data [2603.17009]. Its instrumentation included the 1.5 m Mt. Lemmon CSS telescope, which tiled \(145\,\mathrm{deg}^2\) total at \(0.5\) d and \(1.8\) d post-merger to a \(5\sigma\) depth of \(\approx 20.7\) mag in \(G_{\rm AB}\); the 0.82 m T80-South at CTIO, which observed \(23\) fields covering \(\sim 45\,\mathrm{deg}^2\) at \(0.5\) d with a \(3\sigma\) limit \(i>21.2\) mag; and the 0.4 m PROMPT telescopes in the DLT40 survey, which observed \(184\) galaxies in a galaxy-targeted program [2603.17009].

That search also ingested candidates reported by GOTO, BlackGEM, ATLAS, ZTF, DECam, GRANDMA, Rubin-LSST, WFST, J-GEM, SVOM/VT, LOT, GECKO/KMTNet, and ATLAS, along with X-ray searches by Fermi-GBM, Glowbug, MAXI/GSC, Swift-XRT, and EP-FXT [2603.17009]. In total, \(248\) candidates within \(\le 10\) d post-GW were ingested, including \(67\) from Rubin-LSST [2603.17009].

For vetting, that work introduced a modular scoring framework over four hypothesized counterparts,
\[
T\in\{\mathrm{KN},\mathrm{KN\mbox{-}in\mbox{-}SN},\mathrm{super\mbox{-}KN},\mathrm{AGN\mbox{-}flare}\},
\]
with an overall score
\[
S_T = S_{2D}\times S_{\mathrm{dist},T}\times S_{\mathrm{PSMPC},T}\times S_{\mathrm{AGN},T}\times S_{\mathrm{phot},T}.
\]
The distance term is
\[
S_{\mathrm{dist}}=\int P_{\mathrm{GW}}(D)\,P_{\mathrm{cand}}(D)\,dD,
\]
and the point-source/minor-planet score factorizes as \(S_{\mathrm{PSMPC}} = S_{\mathrm{PS}}\times S_{\mathrm{MPC}}\) [2603.17009]. The photometric term is the product of four binary factors for predetections, peak luminosity, rise time, and decay rate, each equal to \(1\) if within bounds and \(0.1\) otherwise, yielding \(0.0001 \le S_{\mathrm{phot}} \le 1\) [2603.17009].

The allowed photometric ranges differ by transient class. For KN, the criteria are no predetections, \(L_{\mathrm{peak}} < 10^{43}\,\mathrm{erg/s}\), \(t_{\mathrm{peak}}<4\,\mathrm{d}\), and \(\Delta m/dt > 0.1\,\mathrm{mag/d}\). For KN-in-SN, predetections up to \(\le 1\) d before merger are allowed, \(5\times10^{41} < L_{\mathrm{peak}} < 10^{44}\,\mathrm{erg/s}\), \(t_{\mathrm{peak}} < 35\,\mathrm{d}\), and \(-2.0 < \Delta m/dt < +0.1\,\mathrm{mag/d}\). For super-KN, predetections up to \(\le 1\) d are allowed, \(10^{41} < L_{\mathrm{peak}} < 10^{43}\,\mathrm{erg/s}\), \(10 < t_{\mathrm{peak}} < 70\,\mathrm{d}\), and no \(\Delta m\) bound is imposed [2603.17009].

## 3. Kilonova non-detection and model exclusion

No credible kilonova counterpart was found in the FTW, ZTF, and DECam follow-up [2605.10940]. The observational depths and cadences are explicitly quantified. FTW/3KK reached approximately \(r\sim22\)–\(23\) mag, \(i\sim21\)–\(22\) mag, and \(J\sim20\)–\(21\) mag in a galaxy-targeted nightly cadence over three nights spanning \(t\sim0.11\)–\(2.15\) d. ZTF reached \(g\sim19.0\) mag and \(r\sim20.1\) mag at \(300\) s, with nightly \(30\) s exposures in \(g,r\) from \(t+11\) d onward. DECam reached \(g\sim23.4\) mag and \(i\sim22.7\) mag per \(30\) s over six epochs spanning \(t\approx1.4\)–\(18\) d [2605.10940].

The non-detection was translated into quantitative model exclusions using the 3D radiative-transfer POSSIS grid, comprising \(3072\) ejecta combinations and \(11\) viewing angles with parameters \(M_{\rm dyn}\), \(M_{\rm wind}\), \(v_{\rm dyn}\), \(v_{\rm wind}\), and \(X_{\rm lan}\) [2605.10940]. For each model light curve \(m_{\mathrm{model}}(t)\), models were excluded at observation time \(t_{\mathrm{obs}}\) if
\[
m_{\mathrm{model}}(t_{\mathrm{obs}}) < m_{\lim}\;(5\sigma).
\]
Under this procedure, ZTF excludes \(42\%\) of the grid, DECam excludes \(68\%\), and FTW excludes \(92\%\) [2605.10940]. When weighted by sky-coverage \(f_{\rm cov}\), using ZTF \(=43\%\), DECam \(=20\%\), and FTW targeted \(\sim0.1\%\), the combined exclusion probability is \(\sim29\%\) [2605.10940].

The same study summarizes the result as ruling out \(\sim30\%\) of plausible kilonova models across \(56\%\) of the sky [2605.10940]. The most strongly constrained ejecta configurations are those with \(M_{\rm wind}\gtrsim0.05\,M_\odot\) or high \(X_{\rm lan}\), which are described as largely ruled out [2605.10940].

Another counterpart-search paper independently reports no likely counterpart after vetting \(248\) candidates [2603.17009]. Its score distributions show that \(118\) candidates had \(S_T \ge 0.01\) for at least one transient class, with \(77\) scoring as KN, \(96\) as KN-in-SN, \(66\) as super-KN, and \(1\) as AGN-flare [2603.17009]. It further states that \(163\) candidates had \(S_{\mathrm{dist}} \le 0.10\), indicating host-galaxy distances incompatible with the GW distance, and that spectroscopy of nine candidates classified them all as SNe Ia or II at \(z \gg 0.03\), or with host redshifts outside \(93 \pm 27\) Mpc, effectively ruling them out [2603.17009].

A plausible implication is that the absence of a classical kilonova counterpart does not eliminate all nonstandard channels, but it does significantly restrict bright kilonova parameter space under the modeled ejecta assumptions.

## 4. The superkilonova channel and the SN 2025adtq association

One proposed formation channel for sub-solar neutron-star mergers is the disk-fragmentation or “superkilonova” scenario, attributed to Metzger et al. 2024 and Lerner et al. 2025 in the follow-up analysis [2605.10940]. In this picture, a rapidly rotating collapsar disk fragments into sub-solar neutron stars which merge a time \(\Delta t \sim\) hours to days after core collapse [2605.10940].

The relevant inspiral timescale is written using the Peters formula:
\[
t_{\mathrm{GW}} = \frac{5\,c^5 a^4}{256\,G^3\,m_1\,m_2\,(m_1+m_2)}.
\]
For equal \(m_{\mathrm{NS}}\sim0.3\,M_\odot\) at orbital radius \(r\approx300\,r_g\) around a \(\sim10\,M_\odot\) black hole, the quoted timescales are \(t_{\mathrm{NS\mbox{-}NS}}\lesssim3.6\) d and \(t_{\mathrm{NS\mbox{-}BH}}\lesssim11\) d [2605.10940].

Motivated by this timescale, the search examined ZTF, FTW, and DECam data in the \(t_0-4\) d window prior to the GW time [2605.10940]. One candidate survived vetting: SN 2025adtq, spectroscopically classified as Type IIb via SALT, HET, P200, and Keck spectra [2605.10940]. Hydrodynamical modeling with SuperSNEC yielded ejecta mass \(M_{\mathrm{ej}}=2.14\,M_\odot\), kinetic energy \(E_{\mathrm{kin}} = 0.6\times10^{51}\) erg, nickel mass \(\sim0.075\,M_\odot\), radius \(\simeq9\,R_\odot\), and an explosion epoch
\[
t_0(\mathrm{SN})=\mathrm{MJD}\,60988.7\pm1,
\]
which is approximately \(2\) d before S251112cm [2605.10940].

The spatial association statistic is defined through the overlap integral
\[
\mathcal{I}=\int \frac{p(d_L\mid \alpha,\delta)\,p(\alpha,\delta)}{\pi(\alpha,\delta)}\,d\alpha\,d\delta,
\]
with \(\log_{10}\mathcal{I}\approx4.8\) for SN 2025adtq [2605.10940]. The chance-coincidence calculation uses
\[
\lambda_{\mathrm{SN}} = R_{\mathrm{SN}}\,V_{\mathrm{eff}}\,\Delta t,
\qquad
P_{\mathrm{coincidence}} = 1-e^{-\lambda_{\mathrm{SN}}}\approx0.045,
\]
with \(V_{\mathrm{eff}}=4.9\times10^5\,\mathrm{Mpc}^3\), \(R_{\mathrm{IIb}}\approx0.8\times10^{-5}\,\mathrm{yr}^{-1}\,\mathrm{Mpc}^{-3}\,h_{70}^3\), and \(\Delta t = 4\) d, corresponding to an estimated chance coincidence probability of \(2\)–\(9\%\) [2605.10940].

The same paper identifies SN 2025adtq as the second Type IIb supernova found in spatial and temporal coincidence with a sub-solar mass GW candidate, following the previously reported S250818k/SN 2025ulz association [2605.10940]. This repetition of Type IIb coincidences is one of the principal motivations for taking the superkilonova channel seriously, although the paper characterizes the evidence as suggestive rather than conclusive [2605.10940].

## 5. Joint statistical interpretation of the two Type IIb supernova coincidences

The earlier S250818k/SN 2025ulz association is summarized as having \(\log_{10}\mathcal{I}\approx4.2\) and \(P_{\mathrm{coincidence}}\approx12\%\) [2605.10940]. A joint analysis across the two events introduces an unconditional false-alarm probability,
\[
\mathrm{FAP}_{\mathrm{uncond}}=
P\!\left(\log_{10}\mathcal{I}\ge \log_{10}\mathcal{I}_{\mathrm{SN}} \mid H_{\mathrm{null}}\right),
\]
and a true-positive rate under the association hypothesis,
\[
\mathrm{TPR}=
P\!\left(\log_{10}\mathcal{I}\le \log_{10}\mathcal{I}_{\mathrm{SN}} \mid H_{\mathrm{assoc}}\right).
\]
For SN 2025adtq, \(\mathrm{FAP}_{\mathrm{uncond}}=1.93\%\) and \(\mathrm{TPR}=5.14\%\); for SN 2025ulz, \(\mathrm{FAP}_{\mathrm{uncond}}=7.10\%\) and \(\mathrm{TPR}=11.49\%\) [2605.10940].

Defining the unconditional odds \(\lambda_{\mathrm{uncond}} = \mathrm{TPR}/\mathrm{FAP}\), the joint result for two events is \(\lambda_{\mathrm{uncond}}^2 \approx 4.32\), corresponding to a joint false-alarm probability of approximately \(0.14\%\), or about \(3\sigma\) [2605.10940]. The same paper also states the formula for the joint false alarm under independence,
\[
P(C_N) = [\mathrm{FAP}]^N.
\]

However, the interpretation changes under conditioning on the existence of at least one coincidence. When conditioned on “\(\ge 1\) coincidence,” the odds reverse, with \(\lambda_{\mathrm{cond}} \approx 0.041\), indicating that the data are more consistent with pure chance if coincidences are assumed [2605.10940]. The paper therefore states that, while the unconditional joint odds are \(\sim4.3{:}1\) in favor of association, the conditional analysis allows a chance-coincidence interpretation [2605.10940].

This duality is central to current discussion of S251112cm. A common misconception would be to read the \(\sim3\sigma\) unconditional result as dispositive evidence for a new channel. The literature does not support that conclusion. Instead, it emphasizes that the evidence is suggestive but inconclusive, and that the interpretive weight depends strongly on the conditioning of the statistical question [2605.10940].

## 6. Primordial black hole interpretations

A distinct line of research treats S251112cm as a possible primordial black hole merger. One analysis examines a broad PBH mass function motivated by the Quantum Chromodynamics epoch and normalized over \(10^{-6}\)–\(4\times10^8\,M_\odot\) such that
\[
\int \left(\frac{df_{\mathrm{PBH}}}{d\ln M}\right)d\ln M = f_{\mathrm{PBH}} = 0.339
\]
[2602.21295]. In that framework, the comoving number density per mass bin is
\[
n(M)\simeq \ln10\;\frac{\rho^{\rm loc}_{\rm DM}}{M}\,\frac{df_{\mathrm{PBH}}}{d\ln M}\,\Delta\ln M,
\]
with \(\rho_{\mathrm{DM}}^{\mathrm{loc}}=\delta_{\mathrm{loc}}\rho_c\Omega_{\mathrm{DM}}\) [2602.21295].

That work adopts a late-Universe GW-capture channel and gives the capture cross section
\[
\bar\sigma(M_1,M_2;v)
= 2\pi\!\left(\frac{85\pi}{6\sqrt2}\right)^{2/7}
\frac{G^2 M_{\rm tot}^{10/7}(M_1 M_2)^{2/7}}{c^{10/7}\,v^{18/7}},
\]
leading to an intrinsic local merger-rate density
\[
\tau_L(M_1,M_2)
= n(M_1)\,n(M_2)\,\bar\sigma(M_1,M_2;v)\,v
\propto \delta_{\rm loc}^2\,v^{-11/7}\,f_{\mathrm{PBH},1}\,f_{\mathrm{PBH},2}\,M_{\rm tot}^{10/7}(M_1M_2)^{5/7}.
\]
The effective comoving detection volume is \(V_{\mathrm{det}} = (4\pi/3)\,r_{\mathrm{det}}^3\), and the detected rate per mass pair is \(R_{\mathrm{det}}(M_1,M_2)=\tau_L\,V_{\mathrm{det}}\) [2602.21295].

Integrating over the chirp-mass region \(0.1 \le \mathcal{M}_c \le 0.87\,M_\odot\) and mass ratios \(0.05 \le q \le 1\), that model predicts \(R_{\mathrm{tot}}=0.8\,\mathrm{yr}^{-1}\) for O3b sensitivity with fiducial \(f_{\mathrm{PBH}}=0.339\), \(\delta_{\mathrm{loc}}\sim3\times10^5\), and \(v\sim \sqrt2\times250\,\mathrm{km/s}\) [2602.21295]. A single detection in \(4.35\) yr implies an observed sub-solar rate
\[
R_{\mathrm{obs}} = 0.23^{+0.86}_{-0.218}\,\mathrm{yr}^{-1}\quad(95\%\ \mathrm{C.L.}),
\]
and the paper states that the model prediction lies within the \(95\%\) Poisson confidence bounds for one event, \(0.012\)–\(1.09\,\mathrm{yr}^{-1}\) [2602.21295]. Because \(R_{\mathrm{det}}\propto f_{\mathrm{PBH}}^2\), it derives a lower bound
\[
f_{\mathrm{PBH},\min} \simeq 0.04
\]
if the trigger is confirmed as astrophysical [2602.21295].

A second PBH study instead assumes a monochromatic mass function,
\[
\psi(m)=\delta(m-M_{\mathrm{PBH}}),
\qquad
\int dm\,\psi(m)=f_{\mathrm{PBH}},
\]
and uses an early-Universe PBH binary merger-rate formalism [2603.25795]. Its present-day equal-mass monochromatic rate is
\[
R_0 \simeq
3.14\times10^6\;\frac{\mathrm{Gpc}^{-3}}{\mathrm{yr}}\;
\frac{f_{\mathrm{PBH}}^{53/37}}{S}
\left(\frac{M_{\mathrm{PBH}}}{M_\odot}\right)^{-32/37},
\]
with suppression factor
\[
S\simeq0.24\,[1+2.3\,(\sigma_M/f_{\mathrm{PBH}})^2]^{-21/74},
\qquad \sigma_M\simeq0.005,
\]
and the detection probability
\[
P(M_{\mathrm{PBH}})=1-\exp[-R_0\langle VT\rangle].
\]
The effective sensitivity is taken to scale as \(\langle VT\rangle\propto M_{\mathrm{PBH}}^{2.34}\), corresponding to \(\alpha\simeq0.78\) in the O3 injection-based fit [2603.25795].

Under a “relaxed” scenario for PBH abundance constraints, that study finds \( \mathcal{P}\approx1\) for \(M_{\mathrm{PBH}}\gtrsim0.7\,M_\odot\) with O3 sensitivity, and unity already at \(M_{\mathrm{PBH}}\gtrsim0.5\,M_\odot\) for projected O4a sensitivity. Under a “conservative” scenario, the probability remains \(\sim \mathcal{O}(0.3\text{--}0.6)\) across \(0.2\)–\(0.8\,M_\odot\) [2603.25795]. Both PBH papers therefore conclude that a PBH origin for S251112cm is viable within current bounds, while emphasizing that the inference is not conclusive because of astrophysical and instrumental uncertainties [2602.21295] [2603.25795].

## 7. Scientific implications and unresolved questions

The literature converges on several points. First, no classical kilonova counterpart has been identified, despite rapid optical, near-infrared, and X-ray follow-up [2605.10940] [2603.17009]. Second, S251112cm occupies a parameter regime that is difficult to reconcile with standard stellar evolution, which is why both nonstandard astrophysical scenarios and PBH scenarios have received sustained attention [2602.21295] [2603.25795]. Third, the superkilonova interpretation is currently supported only by suggestive circumstantial evidence, especially the temporal and spatial association of Type IIb supernovae with two sub-solar GW candidates [2605.10940].

The most explicit future tests are enumerated in the superkilonova follow-up study. Robust confirmation would require detection of the predicted NS–BH merger in the ensuing days, late-time infrared or nebular spectroscopy such as with JWST to search for \(r\)-process lines, and systematic offline searches for sub-solar mergers near classified Type IIb supernovae [2605.10940]. The same paper argues that future wide-field synoptic surveys, including LSST and Argus, together with extended GW observing duty cycles, will be critical for establishing or rejecting the superkilonova channel [2605.10940].

Complementary recommendations from the counterpart-vetting study include extending follow-up beyond \(10\) d in optical and near-infrared bands to capture slower transients such as super-KN and KN-in-SN, incorporating deep galaxy catalogs such as DESI DR1+ for host matching, deploying real-time vetting tools such as TROVE, and developing refined models of KN-in-SN and super-KN light curves and spectra [2603.17009].

For PBH interpretations, the principal unresolved issues are the sensitivity of abundance constraints to Galactic halo and disk modeling, the treatment of microlensing candidates, the extrapolation of detector sensitivity into the sub-solar regime, and assumptions about the dominant binary-formation channel or mass function shape [2602.21295] [2603.25795]. This suggests that repeated sub-solar detections, rather than a single event, will be required to discriminate between a PBH population and rare astrophysical formation channels.

In its current status, S251112cm remains an unusually consequential but unresolved trigger: a sub-solar compact binary candidate with no confirmed electromagnetic counterpart, nontrivial kilonova constraints, a possible but inconclusive Type IIb-supernova association, and a PBH interpretation that is viable within existing bounds but not uniquely favored [2605.10940] [2602.21295] [2603.25795].

Source: https://www.emergentmind.com/topics/s251112cm