# Search for sub-solar mass ultracompact binaries in Advanced LIGO's second observing run

## TL;DR

This paper reports a search for sub-solar mass ultracompact binaries using Advanced LIGO's second observing run data, finding no candidates and setting upper limits on their merger rates and dark matter contributions.

## Contribution

The study extends previous searches by including component spin effects and provides new constraints on the merger rates and dark matter fraction of sub-solar mass black hole binaries.

## Key findings

- No viable sub-solar mass binary candidates detected.
- Upper limits on merger rates: <3.7 x 10^5 Gpc^-3 yr^-1 for 0.2 solar masses.
- Constraints on dark matter fraction: less than 16% for 0.2 solar mass black holes.

## Abstract

We present an Advanced LIGO and Advanced Virgo search for sub-solar mass ultracompact objects in data obtained during Advanced LIGO's second observing run. In contrast to a previous search of Advanced LIGO data from the first observing run, this search includes the effects of component spin on the gravitational waveform. We identify no viable gravitational wave candidates consistent with sub-solar mass ultracompact binaries with at least one component between 0.2 - 1.0 solar masses. We use the null result to constrain the binary merger rate of (0.2 solar mass, 0.2 solar mass) binaries to be less than 3.7 x 10^5 Gpc^-3 yr^-1 and the binary merger rate of (1.0 solar mass, 1.0 solar mass) binaries to be less than 5.2 x 10^3 Gpc^-3 yr^-1. Sub-solar mass ultracompact objects are not expected to form via known stellar evolution channels, though it has been suggested that primordial density fluctuations or particle dark matter with cooling mechanisms and/or nuclear interactions could form black holes with sub-solar masses. Assuming a particular primordial black hole formation model, we constrain a population of merging 0.2 solar mass black holes to account for less than 16% of the dark matter density and a population of merging 1.0 solar mass black holes to account for less than 2% of the dark matter density. We discuss how constraints on the merger rate and dark matter fraction may be extended to arbitrary black hole population models that predict sub-solar mass binaries.