Search for CBCs with SSM Components in Data from The First Part of LVK Fourth Observing Run

Abstract: Star evolution models predict the lightest compact objects in the universe to have masses greater than that of the Sun. Nonetheless, alternative scenarios could lead to the formation of sub-solar mass (SSM) compact objects, such as primordial black holes (PBHs). The LIGO-Virgo-KAGRA Collaboration (LVK) has performed a search for gravitational-wave (GW) signals from compact binary coalescences (CBCs) including at least one SSM component during the first part of their fourth observing run (O4a), reporting no statistically significant candidate. This non-detection sets upper limits on the merger rate of such systems, which can be used to constrain PBH formation models and the fraction of dark matter (DM) in PBHs. For PBH binaries forming at late times, the fraction of DM in PBHs is constrained to be <= 1 for masses above 0.9 M_sun in the case of monochromatic mass functions. In the early-formation scenario, this fraction is limited to $\le 7\%$ at 1 $M_\odot$ and $\le 40\%$ at 0.35 $M_\odot$.

• The paper reports a null detection of gravitational waves from sub-solar mass binary coalescences during LVK's first O4a run, setting new upper limits on merger rates.
• It employs three independent matched-filtering pipelines (GstLAL, MBTA, and PyCBC) to analyze coincident data, enhancing sensitivity across a wide parameter space.
• The derived constraints significantly restrict primordial black hole dark matter scenarios by tightening limits on both early and late binary formation models.

Search for Sub-Solar Mass Compact Binary Coalescences with LVK's O4a Run

Context and Motivation

Traditional astrophysical models posit that compact objects produced through stellar evolution, such as neutron stars and black holes, possess masses strictly above $1~M_\odot$. This threshold arises from the Chandrasekhar limit and mechanisms underlying stellar collapse. Nevertheless, theories invoking exotic formation channels—most notably, Primordial Black Holes (PBHs)—enable the possibility that sub-solar mass (SSM) compact objects exist. Detection of gravitational waves (GWs) from compact binary coalescences (CBCs) with SSM components would provide direct evidence for such alternatives, with significant repercussions for understanding dark matter (DM) composition and PBH formation models.

Experimental Methodology

The LIGO-Virgo-KAGRA (LVK) Collaboration performed a dedicated search for GW signals from CBCs involving SSM components over the first segment of their fourth observing run (O4a). The analysis incorporated coincident data from the two Advanced LIGO detectors, excluding Virgo due to its inactivity during this timeframe.

Three independent pipelines—GstLAL, MBTA, and PyCBC—were deployed, utilizing matched-filtering techniques across a template bank parameter space:

• Redshifted masses $(1+z)m_1 \in [0.2,10.0]~M_\odot$, $(1+z)m_2 \in [0.2,1.0]~M_\odot$,
• Mass ratio $m_2/m_1 \in [0.1,1.0]$,
• Spin magnitudes constrained per mass regime, aligned or anti-aligned with orbital angular momentum.

The search sensitivity was quantified via simulation campaigns as the sensitive spacetime volume $\langle VT \rangle$, tailored for distinct chirp mass bins and population morphologies.

Results and Statistical Analysis

No statistically significant GW candidates consistent with SSM CBCs were identified across all pipelines. Event distributions, as characterized by false-alarm rates (FARs), adhered to noise expectations barring GW230529_181500—a candidate with secondary mass above $1~M_\odot$, precluding an SSM classification.

Upper limits on the merger rates of CBCs containing SSM BHs were derived, employing the loudest-event formalism for Poisson statistics:

$\mathcal{R}_{90,i} = 2.3 / \langle VT \rangle_i$

across chirp mass intervals. Resultant constraints are marginal improvements relative to those from O3, attributed to detector performance enhancements counterbalanced by shortened O4a observing duration.

Constraints on Primordial Black Holes as Dark Matter

The merger-rate upper bounds enabled further inference on PBH scenarios and their DM fractions ($f_{\mathrm{PBH}}$), given model-dependent mappings reliant on PBH mass functions and clustering. Both early binary (EB) and late binary (LB) formation channels were considered for monochromatic mass distributions.

Distinct, strong constraints are established:

• Late-time PBH binaries: $f_{\mathrm{PBH}} \leq 1$ for $m_{\mathrm{PBH}} > 0.9~M_\odot$.
• Early-formation PBH binaries: $(1+z)m_1 \in [0.2,10.0]~M_\odot$0 at $(1+z)m_1 \in [0.2,10.0]~M_\odot$1; $(1+z)m_1 \in [0.2,10.0]~M_\odot$2 at $(1+z)m_1 \in [0.2,10.0]~M_\odot$3.

These numerical results reflect consistency across pipelines and incrementally improve upon previous constraints, further closing parameter space for viable DM fractions in PBHs.

Implications and Future Directions

The null detection of SSM CBCs in O4a critically informs the phenomenology of compact object formation below the Chandrasekhar limit and PBH-based DM models. Practically, this enhances the exclusion regions in DM-PBH parameter space and informs the necessity for more sensitive instruments or longer baselines to probe remaining unconstrained domains.

Theoretically, these results restrict the viability of PBH scenarios, especially those positing PBHs as a majority DM constituent for masses above $(1+z)m_1 \in [0.2,10.0]~M_\odot$4. Further observing runs with increased sensitivity and extended duration are projected to refine these bounds, potentially excluding or revealing novel SSM objects formed via non-stellar mechanisms.

Conclusion

The LVK search over O4a for CBCs with SSM components yielded no significant detections. Upper limits on merger rates and the PBH DM fraction were tightened, enhancing constraints on non-stellar compact objects and PBH DM hypotheses. Continuing observations will progressively challenge alternative formation mechanisms and their role in cosmic DM composition (2604.14095).