S251112cm: Subsolar GW Candidate

• S251112cm is a gravitational-wave candidate featuring a sub-solar chirp mass from a symmetric binary merger, challenging standard stellar evolution.
• Follow-up campaigns using DECam, ZTF, and FTW quantified deep limits that exclude up to 92% of bright kilonova models.
• Population interpretations explore both a nonstandard superkilonova channel and primordial black hole scenarios, each with distinct statistical uncertainties.

Searching arXiv for 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 (Hall et al., 11 May 2026, Vieira et al., 17 Mar 2026, Magaraggia et al., 24 Feb 2026, Haque et al., 26 Mar 2026).

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 (Vieira et al., 17 Mar 2026). 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 (Hall et al., 11 May 2026, Magaraggia et al., 24 Feb 2026). The log-Bayes coherence factor across the three detectors is reported as $\ln B = +6.1$, indicating a coherent multi-detector signal (Vieira et al., 17 Mar 2026).

The luminosity distance is reported as $1$0 Mpc (Vieira et al., 17 Mar 2026, Magaraggia et al., 24 Feb 2026). The source chirp mass is given in the interval $1$1–$1$2 (Hall et al., 11 May 2026, Vieira et al., 17 Mar 2026, Magaraggia et al., 24 Feb 2026), and the probability that the system contains at least one subsolar-mass object is stated as $1$3 or $1$4, depending on the analysis (Vieira et al., 17 Mar 2026, Magaraggia et al., 24 Feb 2026). The LVK quantity $1$5 indicates an $1$6 probability of containing a neutron star in the $1$7–$1$8 range by LVK definition (Vieira et al., 17 Mar 2026).

The chirp mass is related to component masses by

$1$9

with the component masses also expressible in terms of the mass ratio $6.2$0 as

$6.2$1

One summary notes that the chirp-mass bin $6.2$2–$6.2$3 implies component masses $6.2$4, $6.2$5 under reasonable priors (Hall et al., 11 May 2026). Another concludes that, because $6.2$6 and $6.2$7, the most likely interpretation is a roughly symmetric binary of two subsolar-mass objects (Vieira et al., 17 Mar 2026).

Sky-localization estimates differ across analyses. One electromagnetic follow-up study describes a gravitational-wave localization spanning $6.2$8 for the $6.2$9/$1$0 credible regions (Hall et al., 11 May 2026), whereas another PBH-oriented summary quotes a $1$1 area of $1$2 (Magaraggia et al., 24 Feb 2026). 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 $1$3 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 $1$4 of the GW probability, or $1$5 including WFST (Hall et al., 11 May 2026). The facilities and strategies were distinct: FTW/3KK performed galaxy-targeted observations in $1$6 bands to DESI DR1 hosts; ZTF executed snapshot tiling of the northern lobe in $1$7 at $1$8 hr post-alert; and DECam surveyed the southern lobe in $1$9 from $4$0 d post-event (Hall et al., 11 May 2026).

A separate counterpart search combined triggered observations with SAGUARO telescopes, community alerts, and survey data (Vieira et al., 17 Mar 2026). Its instrumentation included the 1.5 m Mt. Lemmon CSS telescope, which tiled $4$1 total at $4$2 d and $4$3 d post-merger to a $4$4 depth of $4$5 mag in $4$6; the 0.82 m T80-South at CTIO, which observed $4$7 fields covering $4$8 at $4$9 d with a $6.2$0 limit $6.2$1 mag; and the 0.4 m PROMPT telescopes in the DLT40 survey, which observed $6.2$2 galaxies in a galaxy-targeted program (Vieira et al., 17 Mar 2026).

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 (Vieira et al., 17 Mar 2026). In total, $6.2$3 candidates within $6.2$4 d post-GW were ingested, including $6.2$5 from Rubin-LSST (Vieira et al., 17 Mar 2026).

For vetting, that work introduced a modular scoring framework over four hypothesized counterparts,

$6.2$6

with an overall score

$6.2$7

The distance term is

$6.2$8

and the point-source/minor-planet score factorizes as $6.2$9 (Vieira et al., 17 Mar 2026). The photometric term is the product of four binary factors for predetections, peak luminosity, rise time, and decay rate, each equal to $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$0 if within bounds and $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$1 otherwise, yielding $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$2 (Vieira et al., 17 Mar 2026).

The allowed photometric ranges differ by transient class. For KN, the criteria are no predetections, $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$3, $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$4, and $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$5. For KN-in-SN, predetections up to $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$6 d before merger are allowed, $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$7, $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$8, and $\mathrm{FAR} \approx 1/6.2\,\mathrm{yr}$9. For super-KN, predetections up to $\sim 1$0 d are allowed, $\sim 1$1, $\sim 1$2, and no $\sim 1$3 bound is imposed (Vieira et al., 17 Mar 2026).

3. Kilonova non-detection and model exclusion

No credible kilonova counterpart was found in the FTW, ZTF, and DECam follow-up (Hall et al., 11 May 2026). The observational depths and cadences are explicitly quantified. FTW/3KK reached approximately $\sim 1$4–$\sim 1$5 mag, $\sim 1$6–$\sim 1$7 mag, and $\sim 1$8–$\sim 1$9 mag in a galaxy-targeted nightly cadence over three nights spanning $4$0–$4$1 d. ZTF reached $4$2 mag and $4$3 mag at $4$4 s, with nightly $4$5 s exposures in $4$6 from $4$7 d onward. DECam reached $4$8 mag and $4$9 mag per $\ln B = +6.1$0 s over six epochs spanning $\ln B = +6.1$1–$\ln B = +6.1$2 d (Hall et al., 11 May 2026).

The non-detection was translated into quantitative model exclusions using the 3D radiative-transfer POSSIS grid, comprising $\ln B = +6.1$3 ejecta combinations and $\ln B = +6.1$4 viewing angles with parameters $\ln B = +6.1$5, $\ln B = +6.1$6, $\ln B = +6.1$7, $\ln B = +6.1$8, and $\ln B = +6.1$9 (Hall et al., 11 May 2026). For each model light curve $1$00, models were excluded at observation time $1$01 if

$1$02

Under this procedure, ZTF excludes $1$03 of the grid, DECam excludes $1$04, and FTW excludes $1$05 (Hall et al., 11 May 2026). When weighted by sky-coverage $1$06, using ZTF $1$07, DECam $1$08, and FTW targeted $1$09, the combined exclusion probability is $1$10 (Hall et al., 11 May 2026).

The same study summarizes the result as ruling out $1$11 of plausible kilonova models across $1$12 of the sky (Hall et al., 11 May 2026). The most strongly constrained ejecta configurations are those with $1$13 or high $1$14, which are described as largely ruled out (Hall et al., 11 May 2026).

Another counterpart-search paper independently reports no likely counterpart after vetting $1$15 candidates (Vieira et al., 17 Mar 2026). Its score distributions show that $1$16 candidates had $1$17 for at least one transient class, with $1$18 scoring as KN, $1$19 as KN-in-SN, $1$20 as super-KN, and $1$21 as AGN-flare (Vieira et al., 17 Mar 2026). It further states that $1$22 candidates had $1$23, indicating host-galaxy distances incompatible with the GW distance, and that spectroscopy of nine candidates classified them all as SNe Ia or II at $1$24, or with host redshifts outside $1$25 Mpc, effectively ruling them out (Vieira et al., 17 Mar 2026).

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 (Hall et al., 11 May 2026). In this picture, a rapidly rotating collapsar disk fragments into sub-solar neutron stars which merge a time $1$26 hours to days after core collapse (Hall et al., 11 May 2026).

The relevant inspiral timescale is written using the Peters formula: $1$27 For equal $1$28 at orbital radius $1$29 around a $1$30 black hole, the quoted timescales are $1$31 d and $1$32 d (Hall et al., 11 May 2026).

Motivated by this timescale, the search examined ZTF, FTW, and DECam data in the $1$33 d window prior to the GW time (Hall et al., 11 May 2026). One candidate survived vetting: SN 2025adtq, spectroscopically classified as Type IIb via SALT, HET, P200, and Keck spectra (Hall et al., 11 May 2026). Hydrodynamical modeling with SuperSNEC yielded ejecta mass $1$34, kinetic energy $1$35 erg, nickel mass $1$36, radius $1$37, and an explosion epoch

$1$38

which is approximately $1$39 d before S251112cm (Hall et al., 11 May 2026).

The spatial association statistic is defined through the overlap integral

$1$40

with $1$41 for SN 2025adtq (Hall et al., 11 May 2026). The chance-coincidence calculation uses

$1$42

with $1$43, $1$44, and $1$45 d, corresponding to an estimated chance coincidence probability of $1$46–$1$47 (Hall et al., 11 May 2026).

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 (Hall et al., 11 May 2026). 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 (Hall et al., 11 May 2026).

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

The earlier S250818k/SN 2025ulz association is summarized as having $1$48 and $1$49 (Hall et al., 11 May 2026). A joint analysis across the two events introduces an unconditional false-alarm probability,

$1$50

and a true-positive rate under the association hypothesis,

$1$51

For SN 2025adtq, $1$52 and $1$53; for SN 2025ulz, $1$54 and $1$55 (Hall et al., 11 May 2026).

Defining the unconditional odds $1$56, the joint result for two events is $1$57, corresponding to a joint false-alarm probability of approximately $1$58, or about $1$59 (Hall et al., 11 May 2026). The same paper also states the formula for the joint false alarm under independence,

$1$60

However, the interpretation changes under conditioning on the existence of at least one coincidence. When conditioned on “$1$61 coincidence,” the odds reverse, with $1$62, indicating that the data are more consistent with pure chance if coincidences are assumed (Hall et al., 11 May 2026). The paper therefore states that, while the unconditional joint odds are $1$63 in favor of association, the conditional analysis allows a chance-coincidence interpretation (Hall et al., 11 May 2026).

This duality is central to current discussion of S251112cm. A common misconception would be to read the $1$64 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 (Hall et al., 11 May 2026).

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 $1$65–$1$66 such that

$1$67

(Magaraggia et al., 24 Feb 2026). In that framework, the comoving number density per mass bin is

$1$68

with $1$69 (Magaraggia et al., 24 Feb 2026).

That work adopts a late-Universe GW-capture channel and gives the capture cross section

$1$70

leading to an intrinsic local merger-rate density

$1$71

The effective comoving detection volume is $1$72, and the detected rate per mass pair is $1$73 (Magaraggia et al., 24 Feb 2026).

Integrating over the chirp-mass region $1$74 and mass ratios $1$75, that model predicts $1$76 for O3b sensitivity with fiducial $1$77, $1$78, and $1$79 (Magaraggia et al., 24 Feb 2026). A single detection in $1$80 yr implies an observed sub-solar rate

$1$81

and the paper states that the model prediction lies within the $1$82 Poisson confidence bounds for one event, $1$83–$1$84 (Magaraggia et al., 24 Feb 2026). Because $1$85, it derives a lower bound

$1$86

if the trigger is confirmed as astrophysical (Magaraggia et al., 24 Feb 2026).

A second PBH study instead assumes a monochromatic mass function,

$1$87

and uses an early-Universe PBH binary merger-rate formalism (Haque et al., 26 Mar 2026). Its present-day equal-mass monochromatic rate is

$1$88

with suppression factor

$1$89

and the detection probability

$1$90

The effective sensitivity is taken to scale as $1$91, corresponding to $1$92 in the O3 injection-based fit (Haque et al., 26 Mar 2026).

Under a “relaxed” scenario for PBH abundance constraints, that study finds $1$93 for $1$94 with O3 sensitivity, and unity already at $1$95 for projected O4a sensitivity. Under a “conservative” scenario, the probability remains $1$96 across $1$97–$1$98 (Haque et al., 26 Mar 2026). 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 (Magaraggia et al., 24 Feb 2026, Haque et al., 26 Mar 2026).

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 (Hall et al., 11 May 2026, Vieira et al., 17 Mar 2026). 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 (Magaraggia et al., 24 Feb 2026, Haque et al., 26 Mar 2026). 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 (Hall et al., 11 May 2026).

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 $1$99-process lines, and systematic offline searches for sub-solar mergers near classified Type IIb supernovae (Hall et al., 11 May 2026). 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 (Hall et al., 11 May 2026).

Complementary recommendations from the counterpart-vetting study include extending follow-up beyond $6.2$00 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 (Vieira et al., 17 Mar 2026).

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 (Magaraggia et al., 24 Feb 2026, Haque et al., 26 Mar 2026). 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 (Hall et al., 11 May 2026, Magaraggia et al., 24 Feb 2026, Haque et al., 26 Mar 2026).