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

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 and black holes are not expected to form via standard stellar evolution, and their observation would signify a new class of astrophysical objects or the discovery of a component of dark matter. Our search covers binaries with primary masses 0.1 to 2 $M_{\odot }$ and secondary masses 0.1 to 1 $M_{\odot }$. We explicitly incorporate tidal effects up to $7\times 10^5$ for extremely low mass neutron stars. Due to the recent development of efficient ratio filter de-chirping frameworks, this search consisting of 25 million templates is now computationally feasible. No statistically significant candidates are identified. We place a $90\%$ confidence upper limit on the merger rate ${\mathcal{R}}_{90}$ for sub-solar mass black holes to be $<2.5\times 10^4{\text{Gpc}}^{-3}{\text{yr}}^{-1}$ for a chirp mass of 0.2 $M_{\odot }$. We place the first constraints for binary neutron stars with tidal deformabilities up to $\sim 7\times 10^5$ and improve the merger rate estimate by a factor $\sim 3$ in comparison to previous O3 tidal searches for tidal deformabilities $<10^4$. 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-O3) combined. We constrain the effective local dark matter fraction to be ${\tilde{f}}_{\text{PBH}}<0.5\%$ for 0.4 $M_{\text{PBH}}$, approximately 1.8 times lower than the previous O1-O3 constraints. Given our model assumptions, our local dark matter fraction constraints are 2-10 times lower than the OGLE microlensing survey for $M_{\text{PBH}}\ge 0.25$.

6 pages, 3 figures, 1 table. This version corrects a bug in our previous analysis that incorrectly scaled the SNR of our rates. All results in this manuscript reflect the corrected calculations