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

TL;DR

This paper reports a second Advanced LIGO–Advanced Virgo search for sub-solar mass ultracompact binaries using O2 data with spin-inclusive waveform templates. No convincing sub-solar mass gravitational-wave candidates were found, enabling the tightest merger-rate and dark-matter abundance constraints to date, including specific bounds on equal-mass sub-solar binaries and corresponding primordial black hole dark matter fractions. The authors extend the interpretation to arbitrary populations by demonstrating that rate limits scale with chirp mass and horizon distance, and present a framework to separate observables from model-dependent terms in PBH-dark-matter scenarios. The work strengthens the case that detections, if any, would signal new physics beyond standard stellar evolution, and it provides a scalable methodology for future runs with improved sensitivity. $D_{ ext{horizon}} \propto \mathcal{M}^{5/6} \sqrt{\int_{f_{ ext{min}}}^{f_{ ext{max}}} \frac{f^{-7/3}}{S_n(f)}\,df}$ and $\mathcal{R}(\mathcal{M})$ sensitivity is governed by the chirp mass, informing how limits apply to diverse sub-solar mass populations.

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.

Figures (2)

• Figure 1: The constraint on the merger rate density for equal mass binaries as a function of chirp mass. The two sets of lines show the constraints for the O1 search Abbott:2018oah and the O2 search presented here. The null result from O2 places bounds that are $\sim 3$ times tighter than the O1 results. The majority of this improvement is due to the increased coincident observing time in Advanced LIGO's second observing run ($\sim 118$ days vs. $\sim 48$ days), though the improved sensitivity of the detectors led to an observed physical volume up to $\sim 50 \%$ larger than in O1 for sub-solar mass ultracompact binaries.
• Figure 2: Constraints on the fraction of dark matter comprised of delta-function distributions of primordial black holes ($f_{\mathrm{PBH}} = \rho_{\mathrm{PBH}} / \rho_{\mathrm{DM}}$). Shown here are (pink) Advanced LIGO constraints from the O2 ultracompact binary search presented here (solid), (orange) microlensing constraints provided by the OGLE (solid), EROS (dashed) Tisserand:2006zx, and MACHO (dotted) collaborations Allsman:2000kg, (cyan) dynamical constraints from observations of Segue I (solid) Koushiappas:2017chw and Eridanus II (dashed) Brandt:2016aco dwarf galaxies, and (blue) supernova lensing constraints from the Joint Light-curve Analysis (solid) and Union 2.1 (dashed) datasets Zumalacarregui:2017qqd. There is an inherent population model dependency in each of these constraints. Advanced LIGO and Advanced Virgo results carry an additional dependence on the binary fraction of the black hole population. Advanced LIGO and Advanced Virgo results use the Planck "TT,TE,EE+lowP+lensing+ext" cosmology Ade:2015xua.