Primordial black hole interpretation of the subsolar merger event S251112cm

The LIGO-Virgo-KAGRA (LVK) candidate event S251112cm suggests that at least one component has a subsolar mass. Since such objects cannot be produced through standard stellar evolution, this observation provides a possible indication of nonstandard formation channels. Primordial black holes (PBHs), formed from the collapse of primordial density fluctuations in the early Universe, are a well-motivated candidate. We investigate the interpretation of S251112cm as the merger of two PBHs in the mass range 0.20–1 M ⊙, within the parameter space directly covered by the O4a subsolar search. Combining analytic estimates of the PBH merger rate with current observational constraints on their abundance and the sensitivity of LVK searches, we compute the probability of observing such an event. We consider both a monochromatic PBH mass function and extended lognormal distributions. Within the relaxed constraint scenario, the detection probability can approach unity toward the upper end of the mass range. A lognormal mass distribution of moderate width can enhance the probability relative to the monochromatic case, whereas a broader distribution can instead suppress it. Our results show that a PBH interpretation of S251112cm is compatible with current observational bounds even for a subdominant PBH abundance. Owing to the dependence of our results on astrophysical uncertainties, such as those affecting the constraints on the abundance of PBHs, they cannot be regarded as conclusive with respect to the nature of the detected event. At the same time, our analysis highlights the potential of subsolar gravitational wave events as a probe of PBHs and their contribution to dark matter.