Search for Sub-Solar Mass Binaries in the First Part of LIGO's Fourth Observing Run

TL;DR

The paper tackles the problem of detecting sub-solar mass compact binaries in LIGO's O4a data, motivated by the existence of primordial black holes and low-mass neutron stars as non-standard objects with implications for dark matter and dense-matter physics. It employs a large, tidal-deformability-aware template bank and a PyCBC-based matched-filter pipeline, supplemented by a ratio-filter de-chirping technique to enable long-duration signal processing across $m_1\in[0.1,2]\,M_\odot$, $m_2\in[0.1,1]\,M_\odot$, and $\lambda_{1,2}\in[0,7\times10^5]$, for waveform durations up to $512$ s and frequencies up to 800 Hz. No confident detections are found, but the analysis yields stringent upper limits on the local merger rate $R_{90}$ as a function of chirp mass $\mathcal{M}$ and tidal deformability $\tilde{\Lambda}$, with O4a improving sensitivity by more than a factor of two over previous runs and extending tidal constraints to $\tilde{\Lambda}\sim 7\times10^5$. In addition to constraining SSM binaries, the work places bounds on the PBH dark-matter fraction $\tilde{f}_{\rm PBH}$ across several masses, providing valuable constraints on non-standard formation scenarios and guiding future search strategies on next-generation detectors.

Abstract

We report the first results of a sub-solar mass compact binary search using the data from the first part of the fourth observing run (O4a) of the Advanced LIGO detectors. Sub-solar mass neutron stars or primordial black holes are not expected to form via standard stellar evolution, and their observation would signify a new class of astrophysical object or the discovery of dark matter. Our search covers binaries with primary masses 0.1 to 2 $\textrm{M}_\odot$ and secondary masses 0.1 to 1 $\textrm{M}_\odot$. We explicitly incorporate tidal effects up to $7\times10^5$ for extremely low mass neutron stars. No statistically significant candidates are identified. The advanced sensitivity of the O4a run enables an improvement in the sub-solar mass black hole merger rate limits by more than $2 \times$ over the previous three observing runs (O1-O3b). We place a $90\%$ confidence upper limit on the merger rate $\mathcal{R}_{90}$ for sub-solar mass black holes to be $< 1.77\times10^4 \textrm{Gpc}^{-3} \textrm{yr}^{-1}$ for a chirp mass of 0.2 $\textrm{M}_\odot$. We place the first constraints for binary neutron stars with tidal deformabilities up to $\sim 7\times10^5$ and improve the merger rate estimate by a factor $\sim 4$ in comparison to previous O3 tidal searches for tidal deformabilities $< 10^4$. We further constrain the fraction of dark matter composed of primordial black hole $f_{\rm PBH}< 2\%$ for a chirp mass of 0.1 $\textrm{M}_\odot$. Our results significantly expand the observational search space for sub-solar binaries and provide rigorous constraints on the local abundance of compact objects that may arise from non-standard formation mechanisms.

Figures (3)

• Figure 1: Top: The parameter space of searches as a function of the effective tidal deformability $\tilde{\Lambda}$2010PhRvD..81l3016H and chirp mass $\mathcal{M}$ for previous tidal searches. The teal and yellows regions are the parameter space targeted in this work. Our previous O3 tidal search is depicted in yellow which directly overlaps which this search 2025ApJ...984...61K. The orange region indicates the range explored by the 4-OGC non-eccentric compact binary search Nitz:2021zwj, and the gray region denotes the tidal Love number search Chia:2023tle. Bottom: The parameter ranges spanned by our search, including component masses, spins, and tidal deformabilities. Waveforms were generated with a maximum duration of 512 seconds, a minimum frequency of 20 Hz, and a maximum frequency of 800 Hz. The bank was constructed to recover at least $95\%$ of the optimal signal-to-noise ratio.
• Figure 2: Upper limit of the merger rate at $90\%$ confidence as a function of chirp mass. The red line represents the sensitivity for all tidal deformations covered by our search; we find that our search achieves equal sensitivity as a function of tidal deformability. The blue line represents the cumulative O1-O4a constraint on the black hole merger rate. Prior BBH constraints incorporating O1-O3b data from nitzsubsolar and the latest LVK sub-solar search ligosubsolar are depicted in the dashed orange line and dotted gray line respectively.
• Figure 3: Constraints on the PBH DM fraction $\tilde{f}_{\rm PBH}$ as a function of chirp mass $\mathcal{M}$. The blue curve represents the cumulative constraints from O1-O4a sensitivity combined from the rates in nitzsubsolar. The dashed orange line represents the nitzsubsolar rates for O1-O3b sensitivity and the dotted gray line represents the constrains from the most recent ligosubsolar sub-solar search.