GW170817: Implications for the Stochastic Gravitational-Wave Background from Compact Binary Coalescences

TL;DR

The total background may be detectable with a signal-to-noise-ratio of 3 after 40 months of total observation time, based on the expected timeline for Advanced LIGO and Virgo to reach their design sensitivity.

Abstract

The LIGO Scientific and Virgo Collaborations have announced the first detection of gravitational waves from the coalescence of two neutron stars. The merger rate of binary neutron stars estimated from this event suggests that distant, unresolvable binary neutron stars create a significant astrophysical stochastic gravitational-wave background. The binary neutron star background will add to the background from binary black holes, increasing the amplitude of the total astrophysical background relative to previous expectations. In the Advanced LIGO-Virgo frequency band most sensitive to stochastic backgrounds (near 25 Hz), we predict a total astrophysical background with amplitude $Ω_{\rm GW} (f=25 \text{Hz}) = 1.8_{-1.3}^{+2.7} \times 10^{-9}$ with $90\%$ confidence, compared with $Ω_{\rm GW} (f=25 \text{Hz}) = 1.1_{-0.7}^{+1.2} \times 10^{-9}$ from binary black holes alone. Assuming the most probable rate for compact binary mergers, we find that the total background may be detectable with a signal-to-noise-ratio of 3 after 40 months of total observation time, based on the expected timeline for Advanced LIGO and Virgo to reach their design sensitivity.

Figures (2)

• Figure 1: The left panel shows the predicted median background for the BNS (red) and BBH (green) models described in the text, the total combined background (blue), and the Poisson error bars (grey shaded region) for the total background. We also show expected PI curves for observing runs O2, O3, and design sensitivity (see the main text for details about the assumptions made for these observing runs). Virgo is included in O3 and beyond. The PI curves for O3 and beyond cross the Poisson error region, indicating the possibility of detecting this background or placing interesting upper limits on the evolution of the binary merger rates with redshift. In the right panel, we plot the signal-to-noise ratio as a function of cumulative observing time for the median total background (blue curve) and associated uncertainty (shaded region). The median of the predicted total background can be detected with ${\rm SNR}=3$ after 40 months of observation time, with LIGO-Virgo reaching design sensitivity (2022 -- 2024). The markers indicate the transition between observing runs. We only show 12 months of the Design phase for clarity, however this phase is 24 months long for the calculation of the PI curves (see prospects_2016).
• Figure 2: We present a simulated time series of duration $10^4$ seconds illustrating the character of the BBH and BNS signals in the time domain. In red we show a simulated BNS background corresponding to the median rate as shown in Figure \ref{['fig:sens']}, and in green we display the median BBH background. We do not show any detector noise, and do not remove some loud and close events that would be detected individually. The region in the black box, from 1800 -- 2600 seconds, is shown in greater detail in the inset. The BNS time series is continuous as it consists of a superposition of overlapping signals. On the other hand the BBH background (in green) is popcorn-like, and the signals do not overlap. Remarkably, even though the backgrounds have very different structure in the time domain, the energy in both backgrounds are comparable below 100 Hz, as seen in Figure \ref{['fig:sens']}.